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This is dash.info, produced by makeinfo version 6.5 from dash.texi.
This manual is for Dash version 2.17.0.
Copyright © 2012–2021 Free Software Foundation, Inc.
Permission is granted to copy, distribute and/or modify this
document under the terms of the GNU Free Documentation License,
Version 1.3 or any later version published by the Free Software
Foundation; with the Invariant Sections being “GNU General Public
License,” and no Front-Cover Texts or Back-Cover Texts. A copy of
the license is included in the section entitled “GNU Free
Documentation License”.
INFO-DIR-SECTION Emacs
START-INFO-DIR-ENTRY
* Dash: (dash.info). A modern list library for GNU Emacs.
END-INFO-DIR-ENTRY

File: dash.info, Node: Top, Next: Installation, Up: (dir)
Dash
****
This manual is for Dash version 2.17.0.
Copyright © 2012–2021 Free Software Foundation, Inc.
Permission is granted to copy, distribute and/or modify this
document under the terms of the GNU Free Documentation License,
Version 1.3 or any later version published by the Free Software
Foundation; with the Invariant Sections being “GNU General Public
License,” and no Front-Cover Texts or Back-Cover Texts. A copy of
the license is included in the section entitled “GNU Free
Documentation License”.
* Menu:
* Installation:: Installing and configuring Dash.
* Functions:: Dash API reference.
* Development:: Contributing to Dash development.
Appendices
* FDL:: The license for this documentation.
* GPL:: Conditions for copying and changing Dash.
* Index:: Index including functions and macros.
— The Detailed Node Listing —
Installation
* Using in a package:: Listing Dash as a package dependency.
* Fontification of special variables:: Font Lock of anaphoric macro variables.
* Info symbol lookup:: Looking up Dash symbols in this manual.
Functions
* Maps::
* Sublist selection::
* List to list::
* Reductions::
* Unfolding::
* Predicates::
* Partitioning::
* Indexing::
* Set operations::
* Other list operations::
* Tree operations::
* Threading macros::
* Binding::
* Side effects::
* Destructive operations::
* Function combinators::
Development
* Contribute:: How to contribute.
* Change log:: List of significant changes by version.
* Contributors:: List of contributors.

File: dash.info, Node: Installation, Next: Functions, Prev: Top, Up: Top
1 Installation
**************
Dash is available on GNU ELPA (https://elpa.gnu.org/) and MELPA
(https://melpa.org/), and can be installed with the standard command
‘package-install’ (*note (emacs)Package Installation::).
‘M-x package-install <RET> dash <RET>’
Install the Dash library.
‘M-x package-install <RET> dash-functional <RET>’
Install an optional library of additional function combinators.
Alternatively, you can just dump ‘dash.el’ or ‘dash-functional.el’ in
your load path somewhere.
* Menu:
* Using in a package:: Listing Dash as a package dependency.
* Fontification of special variables:: Font Lock of anaphoric macro variables.
* Info symbol lookup:: Looking up Dash symbols in this manual.

File: dash.info, Node: Using in a package, Next: Fontification of special variables, Up: Installation
1.1 Using in a package
======================
If you use Dash in your own package, be sure to list it as a dependency
in the library’s headers as follows (*note (elisp)Library Headers::).
;; Package-Requires: ((dash "2.17.0"))
The same goes for the ‘dash-functional.el’ library of function
combinators:
;; Package-Requires: ((dash "2.17.0") (dash-functional "1.2.0"))

File: dash.info, Node: Fontification of special variables, Next: Info symbol lookup, Prev: Using in a package, Up: Installation
1.2 Fontification of special variables
======================================
The autoloaded minor mode ‘dash-fontify-mode’ is provided for optional
fontification of anaphoric Dash variables (‘it’, ‘acc’, etc.) in Emacs
Lisp buffers using search-based Font Lock (*note (emacs)Font Lock::).
In older Emacs versions which do not dynamically detect macros, the
minor mode also fontifies calls to Dash macros.
To automatically enable the minor mode in all Emacs Lisp buffers,
just call its autoloaded global counterpart ‘global-dash-fontify-mode’,
either interactively or from your ‘user-init-file’:
(global-dash-fontify-mode)

File: dash.info, Node: Info symbol lookup, Prev: Fontification of special variables, Up: Installation
1.3 Info symbol lookup
======================
While editing Elisp files, you can use ‘C-h S’ (‘info-lookup-symbol’) to
look up Elisp symbols in the relevant Info manuals (*note (emacs)Info
Lookup::). To enable the same for Dash symbols, use the command
‘dash-register-info-lookup’. It can be called directly when needed, or
automatically from your ‘user-init-file’. For example:
(with-eval-after-load 'info-look
(dash-register-info-lookup))

File: dash.info, Node: Functions, Next: Development, Prev: Installation, Up: Top
2 Functions
***********
This chapter contains reference documentation for the Dash API
(Application Programming Interface). The names of all public functions
defined in the library are prefixed with a dash character (‘-’).
The library also provides anaphoric macro versions of functions where
that makes sense. The names of these macros are prefixed with two
dashes (‘--’) instead of one.
For instance, while the function ‘-map’ applies a function to each
element of a list, its anaphoric counterpart ‘--map’ evaluates a form
with the local variable ‘it’ temporarily bound to the current list
element instead.
;; Normal version.
(-map (lambda (n) (* n n)) '(1 2 3 4))
⇒ (1 4 9 16)
;; Anaphoric version.
(--map (* it it) '(1 2 3 4))
⇒ (1 4 9 16)
The normal version can, of course, also be written as in the
following example, which demonstrates the utility of both versions.
(defun my-square (n)
"Return N multiplied by itself."
(* n n))
(-map #'my-square '(1 2 3 4))
⇒ (1 4 9 16)
* Menu:
* Maps::
* Sublist selection::
* List to list::
* Reductions::
* Unfolding::
* Predicates::
* Partitioning::
* Indexing::
* Set operations::
* Other list operations::
* Tree operations::
* Threading macros::
* Binding::
* Side effects::
* Destructive operations::
* Function combinators::

File: dash.info, Node: Maps, Next: Sublist selection, Up: Functions
2.1 Maps
========
Functions in this category take a transforming function, which is then
applied sequentially to each or selected elements of the input list.
The results are collected in order and returned as a new list.
-- Function: -map (fn list)
Apply FN to each item in LIST and return the list of results. This
function’s anaphoric counterpart is ‘--map’.
(-map (lambda (num) (* num num)) '(1 2 3 4))
⇒ '(1 4 9 16)
(-map #'1+ '(1 2 3 4))
⇒ '(2 3 4 5)
(--map (* it it) '(1 2 3 4))
⇒ '(1 4 9 16)
-- Function: -map-when (pred rep list)
Return a new list where the elements in LIST that do not match the
PRED function are unchanged, and where the elements in LIST that do
match the PRED function are mapped through the REP function.
Alias: ‘-replace-where’
See also: ‘-update-at’ (*note -update-at::)
(-map-when 'even? 'square '(1 2 3 4))
⇒ '(1 4 3 16)
(--map-when (> it 2) (* it it) '(1 2 3 4))
⇒ '(1 2 9 16)
(--map-when (= it 2) 17 '(1 2 3 4))
⇒ '(1 17 3 4)
-- Function: -map-first (pred rep list)
Replace first item in LIST satisfying PRED with result of REP
called on this item.
See also: ‘-map-when’ (*note -map-when::), ‘-replace-first’ (*note
-replace-first::)
(-map-first 'even? 'square '(1 2 3 4))
⇒ '(1 4 3 4)
(--map-first (> it 2) (* it it) '(1 2 3 4))
⇒ '(1 2 9 4)
(--map-first (= it 2) 17 '(1 2 3 2))
⇒ '(1 17 3 2)
-- Function: -map-last (pred rep list)
Replace last item in LIST satisfying PRED with result of REP called
on this item.
See also: ‘-map-when’ (*note -map-when::), ‘-replace-last’ (*note
-replace-last::)
(-map-last 'even? 'square '(1 2 3 4))
⇒ '(1 2 3 16)
(--map-last (> it 2) (* it it) '(1 2 3 4))
⇒ '(1 2 3 16)
(--map-last (= it 2) 17 '(1 2 3 2))
⇒ '(1 2 3 17)
-- Function: -map-indexed (fn list)
Return a new list consisting of the result of (FN index item) for
each item in LIST.
In the anaphoric form ‘--map-indexed’, the index is exposed as
symbol ‘it-index’.
See also: ‘-each-indexed’ (*note -each-indexed::).
(-map-indexed (lambda (index item) (- item index)) '(1 2 3 4))
⇒ '(1 1 1 1)
(--map-indexed (- it it-index) '(1 2 3 4))
⇒ '(1 1 1 1)
-- Function: -annotate (fn list)
Return a list of cons cells where each cell is FN applied to each
element of LIST paired with the unmodified element of LIST.
(-annotate '1+ '(1 2 3))
⇒ '((2 . 1) (3 . 2) (4 . 3))
(-annotate 'length '(("h" "e" "l" "l" "o") ("hello" "world")))
⇒ '((5 "h" "e" "l" "l" "o") (2 "hello" "world"))
(--annotate (< 1 it) '(0 1 2 3))
⇒ '((nil . 0) (nil . 1) (t . 2) (t . 3))
-- Function: -splice (pred fun list)
Splice lists generated by FUN in place of elements matching PRED in
LIST.
FUN takes the element matching PRED as input.
This function can be used as replacement for ‘,@’ in case you need
to splice several lists at marked positions (for example with
keywords).
See also: ‘-splice-list’ (*note -splice-list::), ‘-insert-at’
(*note -insert-at::)
(-splice 'even? (lambda (x) (list x x)) '(1 2 3 4))
⇒ '(1 2 2 3 4 4)
(--splice 't (list it it) '(1 2 3 4))
⇒ '(1 1 2 2 3 3 4 4)
(--splice (equal it :magic) '((list of) (magical) (code)) '((foo) (bar) :magic (baz)))
⇒ '((foo) (bar) (list of) (magical) (code) (baz))
-- Function: -splice-list (pred new-list list)
Splice NEW-LIST in place of elements matching PRED in LIST.
See also: ‘-splice’ (*note -splice::), ‘-insert-at’ (*note
-insert-at::)
(-splice-list 'keywordp '(a b c) '(1 :foo 2))
⇒ '(1 a b c 2)
(-splice-list 'keywordp nil '(1 :foo 2))
⇒ '(1 2)
(--splice-list (keywordp it) '(a b c) '(1 :foo 2))
⇒ '(1 a b c 2)
-- Function: -mapcat (fn list)
Return the concatenation of the result of mapping FN over LIST.
Thus function FN should return a list.
(-mapcat 'list '(1 2 3))
⇒ '(1 2 3)
(-mapcat (lambda (item) (list 0 item)) '(1 2 3))
⇒ '(0 1 0 2 0 3)
(--mapcat (list 0 it) '(1 2 3))
⇒ '(0 1 0 2 0 3)
-- Function: -copy (arg)
Create a shallow copy of LIST.
(fn LIST)
(-copy '(1 2 3))
⇒ '(1 2 3)
(let ((a '(1 2 3))) (eq a (-copy a)))
⇒ nil

File: dash.info, Node: Sublist selection, Next: List to list, Prev: Maps, Up: Functions
2.2 Sublist selection
=====================
Functions returning a sublist of the original list.
-- Function: -filter (pred list)
Return a new list of the items in LIST for which PRED returns
non-nil. Alias: ‘-select’. This function’s anaphoric counterpart
‘--filter’. For similar operations, see also ‘-keep’ (*note
-keep::) and ‘-remove’ (*note -remove::).
(-filter (lambda (num) (= 0 (% num 2))) '(1 2 3 4))
⇒ '(2 4)
(-filter #'natnump '(-2 -1 0 1 2))
⇒ '(0 1 2)
(--filter (= 0 (% it 2)) '(1 2 3 4))
⇒ '(2 4)
-- Function: -remove (pred list)
Return a new list of the items in LIST for which PRED returns nil.
Alias: ‘-reject’. This function’s anaphoric counterpart
‘--remove’. For similar operations, see also ‘-keep’ (*note
-keep::) and ‘-filter’ (*note -filter::).
(-remove (lambda (num) (= 0 (% num 2))) '(1 2 3 4))
⇒ '(1 3)
(-remove #'natnump '(-2 -1 0 1 2))
⇒ '(-2 -1)
(--remove (= 0 (% it 2)) '(1 2 3 4))
⇒ '(1 3)
-- Function: -remove-first (pred list)
Remove the first item from LIST for which PRED returns non-nil.
This is a non-destructive operation, but only the front of LIST
leading up to the removed item is a copy; the rest is LIST’s
original tail. If no item is removed, then the result is a
complete copy. Alias: ‘-reject-first’. This function’s anaphoric
counterpart is ‘--remove-first’. See also ‘-map-first’ (*note
-map-first::), ‘-remove-item’ (*note -remove-item::), and
‘-remove-last’ (*note -remove-last::).
(-remove-first #'natnump '(-2 -1 0 1 2))
⇒ '(-2 -1 1 2)
(-remove-first #'stringp '(1 2 "first" "second"))
⇒ '(1 2 "second")
(--remove-first (> it 3) '(1 2 3 4 5 6))
⇒ '(1 2 3 5 6)
-- Function: -remove-last (pred list)
Return a new list with the last item matching PRED removed.
Alias: ‘-reject-last’
See also: ‘-remove’ (*note -remove::), ‘-map-last’ (*note
-map-last::)
(-remove-last 'even? '(1 3 5 4 7 8 10 11))
⇒ '(1 3 5 4 7 8 11)
(-remove-last 'stringp '(1 2 "last" "second" "third"))
⇒ '(1 2 "last" "second")
(--remove-last (> it 3) '(1 2 3 4 5 6 7 8 9 10))
⇒ '(1 2 3 4 5 6 7 8 9)
-- Function: -remove-item (item list)
Remove all occurrences of ITEM from LIST.
Comparison is done with ‘equal’.
(-remove-item 3 '(1 2 3 2 3 4 5 3))
⇒ '(1 2 2 4 5)
(-remove-item 'foo '(foo bar baz foo))
⇒ '(bar baz)
(-remove-item "bob" '("alice" "bob" "eve" "bob" "dave"))
⇒ '("alice" "eve" "dave")
-- Function: -non-nil (list)
Return all non-nil elements of LIST.
(-non-nil '(1 nil 2 nil nil 3 4 nil 5 nil))
⇒ '(1 2 3 4 5)
-- Function: -slice (list from &optional to step)
Return copy of LIST, starting from index FROM to index TO.
FROM or TO may be negative. These values are then interpreted
modulo the length of the list.
If STEP is a number, only each STEPth item in the resulting section
is returned. Defaults to 1.
(-slice '(1 2 3 4 5) 1)
⇒ '(2 3 4 5)
(-slice '(1 2 3 4 5) 0 3)
⇒ '(1 2 3)
(-slice '(1 2 3 4 5 6 7 8 9) 1 -1 2)
⇒ '(2 4 6 8)
-- Function: -take (n list)
Return a copy of the first N items in LIST. Return a copy of LIST
if it contains N items or fewer. Return nil if N is zero or less.
See also: ‘-take-last’ (*note -take-last::).
(-take 3 '(1 2 3 4 5))
⇒ '(1 2 3)
(-take 17 '(1 2 3 4 5))
⇒ '(1 2 3 4 5)
(-take 0 '(1 2 3 4 5))
⇒ nil
-- Function: -take-last (n list)
Return a copy of the last N items of LIST in order. Return a copy
of LIST if it contains N items or fewer. Return nil if N is zero
or less.
See also: ‘-take’ (*note -take::).
(-take-last 3 '(1 2 3 4 5))
⇒ '(3 4 5)
(-take-last 17 '(1 2 3 4 5))
⇒ '(1 2 3 4 5)
(-take-last 1 '(1 2 3 4 5))
⇒ '(5)
-- Function: -drop (n list)
Return the tail (not a copy) of LIST without the first N items.
Return nil if LIST contains N items or fewer. Return LIST if N is
zero or less. For another variant, see also ‘-drop-last’ (*note
-drop-last::).
(fn N LIST)
(-drop 3 '(1 2 3 4 5))
⇒ '(4 5)
(-drop 17 '(1 2 3 4 5))
⇒ nil
(-drop 0 '(1 2 3 4 5))
⇒ '(1 2 3 4 5)
-- Function: -drop-last (n list)
Return a copy of LIST without its last N items. Return a copy of
LIST if N is zero or less. Return nil if LIST contains N items or
fewer.
See also: ‘-drop’ (*note -drop::).
(-drop-last 3 '(1 2 3 4 5))
⇒ '(1 2)
(-drop-last 17 '(1 2 3 4 5))
⇒ nil
(-drop-last 0 '(1 2 3 4 5))
⇒ '(1 2 3 4 5)
-- Function: -take-while (pred list)
Take successive items from LIST for which PRED returns non-nil.
PRED is a function of one argument. Return a new list of the
successive elements from the start of LIST for which PRED returns
non-nil. This function’s anaphoric counterpart is ‘--take-while’.
For another variant, see also ‘-drop-while’ (*note -drop-while::).
(-take-while #'even? '(1 2 3 4))
⇒ nil
(-take-while #'even? '(2 4 5 6))
⇒ '(2 4)
(--take-while (< it 4) '(1 2 3 4 3 2 1))
⇒ '(1 2 3)
-- Function: -drop-while (pred list)
Drop successive items from LIST for which PRED returns non-nil.
PRED is a function of one argument. Return the tail (not a copy)
of LIST starting from its first element for which PRED returns nil.
This function’s anaphoric counterpart is ‘--drop-while’. For
another variant, see also ‘-take-while’ (*note -take-while::).
(-drop-while #'even? '(1 2 3 4))
⇒ '(1 2 3 4)
(-drop-while #'even? '(2 4 5 6))
⇒ '(5 6)
(--drop-while (< it 4) '(1 2 3 4 3 2 1))
⇒ '(4 3 2 1)
-- Function: -select-by-indices (indices list)
Return a list whose elements are elements from LIST selected as
‘(nth i list)‘ for all i from INDICES.
(-select-by-indices '(4 10 2 3 6) '("v" "e" "l" "o" "c" "i" "r" "a" "p" "t" "o" "r"))
⇒ '("c" "o" "l" "o" "r")
(-select-by-indices '(2 1 0) '("a" "b" "c"))
⇒ '("c" "b" "a")
(-select-by-indices '(0 1 2 0 1 3 3 1) '("f" "a" "r" "l"))
⇒ '("f" "a" "r" "f" "a" "l" "l" "a")
-- Function: -select-columns (columns table)
Select COLUMNS from TABLE.
TABLE is a list of lists where each element represents one row. It
is assumed each row has the same length.
Each row is transformed such that only the specified COLUMNS are
selected.
See also: ‘-select-column’ (*note -select-column::),
‘-select-by-indices’ (*note -select-by-indices::)
(-select-columns '(0 2) '((1 2 3) (a b c) (:a :b :c)))
⇒ '((1 3) (a c) (:a :c))
(-select-columns '(1) '((1 2 3) (a b c) (:a :b :c)))
⇒ '((2) (b) (:b))
(-select-columns nil '((1 2 3) (a b c) (:a :b :c)))
⇒ '(nil nil nil)
-- Function: -select-column (column table)
Select COLUMN from TABLE.
TABLE is a list of lists where each element represents one row. It
is assumed each row has the same length.
The single selected column is returned as a list.
See also: ‘-select-columns’ (*note -select-columns::),
‘-select-by-indices’ (*note -select-by-indices::)
(-select-column 1 '((1 2 3) (a b c) (:a :b :c)))
⇒ '(2 b :b)

File: dash.info, Node: List to list, Next: Reductions, Prev: Sublist selection, Up: Functions
2.3 List to list
================
Functions returning a modified copy of the input list.
-- Function: -keep (fn list)
Return a new list of the non-nil results of applying FN to the
items in LIST.
If you want to select the original items satisfying a predicate use
‘-filter’ (*note -filter::).
(-keep 'cdr '((1 2 3) (4 5) (6)))
⇒ '((2 3) (5))
(-keep (lambda (num) (when (> num 3) (* 10 num))) '(1 2 3 4 5 6))
⇒ '(40 50 60)
(--keep (when (> it 3) (* 10 it)) '(1 2 3 4 5 6))
⇒ '(40 50 60)
-- Function: -concat (&rest lists)
Return a new list with the concatenation of the elements in the
supplied LISTS.
(-concat '(1))
⇒ '(1)
(-concat '(1) '(2))
⇒ '(1 2)
(-concat '(1) '(2 3) '(4))
⇒ '(1 2 3 4)
-- Function: -flatten (l)
Take a nested list L and return its contents as a single, flat
list.
Note that because ‘nil’ represents a list of zero elements (an
empty list), any mention of nil in L will disappear after
flattening. If you need to preserve nils, consider ‘-flatten-n’
(*note -flatten-n::) or map them to some unique symbol and then map
them back.
Conses of two atoms are considered "terminals", that is, they
aren’t flattened further.
See also: ‘-flatten-n’ (*note -flatten-n::)
(-flatten '((1)))
⇒ '(1)
(-flatten '((1 (2 3) (((4 (5)))))))
⇒ '(1 2 3 4 5)
(-flatten '(1 2 (3 . 4)))
⇒ '(1 2 (3 . 4))
-- Function: -flatten-n (num list)
Flatten NUM levels of a nested LIST.
See also: ‘-flatten’ (*note -flatten::)
(-flatten-n 1 '((1 2) ((3 4) ((5 6)))))
⇒ '(1 2 (3 4) ((5 6)))
(-flatten-n 2 '((1 2) ((3 4) ((5 6)))))
⇒ '(1 2 3 4 (5 6))
(-flatten-n 3 '((1 2) ((3 4) ((5 6)))))
⇒ '(1 2 3 4 5 6)
-- Function: -replace (old new list)
Replace all OLD items in LIST with NEW.
Elements are compared using ‘equal’.
See also: ‘-replace-at’ (*note -replace-at::)
(-replace 1 "1" '(1 2 3 4 3 2 1))
⇒ '("1" 2 3 4 3 2 "1")
(-replace "foo" "bar" '("a" "nice" "foo" "sentence" "about" "foo"))
⇒ '("a" "nice" "bar" "sentence" "about" "bar")
(-replace 1 2 nil)
⇒ nil
-- Function: -replace-first (old new list)
Replace the first occurrence of OLD with NEW in LIST.
Elements are compared using ‘equal’.
See also: ‘-map-first’ (*note -map-first::)
(-replace-first 1 "1" '(1 2 3 4 3 2 1))
⇒ '("1" 2 3 4 3 2 1)
(-replace-first "foo" "bar" '("a" "nice" "foo" "sentence" "about" "foo"))
⇒ '("a" "nice" "bar" "sentence" "about" "foo")
(-replace-first 1 2 nil)
⇒ nil
-- Function: -replace-last (old new list)
Replace the last occurrence of OLD with NEW in LIST.
Elements are compared using ‘equal’.
See also: ‘-map-last’ (*note -map-last::)
(-replace-last 1 "1" '(1 2 3 4 3 2 1))
⇒ '(1 2 3 4 3 2 "1")
(-replace-last "foo" "bar" '("a" "nice" "foo" "sentence" "about" "foo"))
⇒ '("a" "nice" "foo" "sentence" "about" "bar")
(-replace-last 1 2 nil)
⇒ nil
-- Function: -insert-at (n x list)
Return a list with X inserted into LIST at position N.
See also: ‘-splice’ (*note -splice::), ‘-splice-list’ (*note
-splice-list::)
(-insert-at 1 'x '(a b c))
⇒ '(a x b c)
(-insert-at 12 'x '(a b c))
⇒ '(a b c x)
-- Function: -replace-at (n x list)
Return a list with element at Nth position in LIST replaced with X.
See also: ‘-replace’ (*note -replace::)
(-replace-at 0 9 '(0 1 2 3 4 5))
⇒ '(9 1 2 3 4 5)
(-replace-at 1 9 '(0 1 2 3 4 5))
⇒ '(0 9 2 3 4 5)
(-replace-at 4 9 '(0 1 2 3 4 5))
⇒ '(0 1 2 3 9 5)
-- Function: -update-at (n func list)
Return a list with element at Nth position in LIST replaced with
‘(func (nth n list))‘.
See also: ‘-map-when’ (*note -map-when::)
(-update-at 0 (lambda (x) (+ x 9)) '(0 1 2 3 4 5))
⇒ '(9 1 2 3 4 5)
(-update-at 1 (lambda (x) (+ x 8)) '(0 1 2 3 4 5))
⇒ '(0 9 2 3 4 5)
(--update-at 2 (length it) '("foo" "bar" "baz" "quux"))
⇒ '("foo" "bar" 3 "quux")
-- Function: -remove-at (n list)
Return a list with element at Nth position in LIST removed.
See also: ‘-remove-at-indices’ (*note -remove-at-indices::),
‘-remove’ (*note -remove::)
(-remove-at 0 '("0" "1" "2" "3" "4" "5"))
⇒ '("1" "2" "3" "4" "5")
(-remove-at 1 '("0" "1" "2" "3" "4" "5"))
⇒ '("0" "2" "3" "4" "5")
(-remove-at 2 '("0" "1" "2" "3" "4" "5"))
⇒ '("0" "1" "3" "4" "5")
-- Function: -remove-at-indices (indices list)
Return a list whose elements are elements from LIST without
elements selected as ‘(nth i list)‘ for all i from INDICES.
See also: ‘-remove-at’ (*note -remove-at::), ‘-remove’ (*note
-remove::)
(-remove-at-indices '(0) '("0" "1" "2" "3" "4" "5"))
⇒ '("1" "2" "3" "4" "5")
(-remove-at-indices '(0 2 4) '("0" "1" "2" "3" "4" "5"))
⇒ '("1" "3" "5")
(-remove-at-indices '(0 5) '("0" "1" "2" "3" "4" "5"))
⇒ '("1" "2" "3" "4")

File: dash.info, Node: Reductions, Next: Unfolding, Prev: List to list, Up: Functions
2.4 Reductions
==============
Functions reducing lists to a single value (which may also be a list).
-- Function: -reduce-from (fn init list)
Reduce the function FN across LIST, starting with INIT. Return the
result of applying FN to INIT and the first element of LIST, then
applying FN to that result and the second element, etc. If LIST is
empty, return INIT without calling FN.
This function’s anaphoric counterpart is ‘--reduce-from’. For
other folds, see also ‘-reduce’ (*note -reduce::) and ‘-reduce-r’
(*note -reduce-r::).
(-reduce-from #'- 10 '(1 2 3))
⇒ 4
(-reduce-from #'list 10 '(1 2 3))
⇒ '(((10 1) 2) 3)
(--reduce-from (concat acc " " it) "START" '("a" "b" "c"))
⇒ "START a b c"
-- Function: -reduce-r-from (fn init list)
Reduce the function FN across LIST in reverse, starting with INIT.
Return the result of applying FN to the last element of LIST and
INIT, then applying FN to the second-to-last element and the
previous result of FN, etc. That is, the first argument of FN is
the current element, and its second argument the accumulated value.
If LIST is empty, return INIT without calling FN.
This function is like ‘-reduce-from’ (*note -reduce-from::) but the
operation associates from the right rather than left. In other
words, it starts from the end of LIST and flips the arguments to
FN. Conceptually, it is like replacing the conses in LIST with
applications of FN, and its last link with INIT, and evaluating the
resulting expression.
This function’s anaphoric counterpart is ‘--reduce-r-from’. For
other folds, see also ‘-reduce-r’ (*note -reduce-r::) and ‘-reduce’
(*note -reduce::).
(-reduce-r-from #'- 10 '(1 2 3))
⇒ -8
(-reduce-r-from #'list 10 '(1 2 3))
⇒ '(1 (2 (3 10)))
(--reduce-r-from (concat it " " acc) "END" '("a" "b" "c"))
⇒ "a b c END"
-- Function: -reduce (fn list)
Reduce the function FN across LIST. Return the result of applying
FN to the first two elements of LIST, then applying FN to that
result and the third element, etc. If LIST contains a single
element, return it without calling FN. If LIST is empty, return
the result of calling FN with no arguments.
This function’s anaphoric counterpart is ‘--reduce’. For other
folds, see also ‘-reduce-from’ (*note -reduce-from::) and
‘-reduce-r’ (*note -reduce-r::).
(-reduce #'- '(1 2 3 4))
⇒ -8
(-reduce #'list '(1 2 3 4))
⇒ '(((1 2) 3) 4)
(--reduce (format "%s-%d" acc it) '(1 2 3))
⇒ "1-2-3"
-- Function: -reduce-r (fn list)
Reduce the function FN across LIST in reverse. Return the result
of applying FN to the last two elements of LIST, then applying FN
to the third-to-last element and the previous result of FN, etc.
That is, the first argument of FN is the current element, and its
second argument the accumulated value. If LIST contains a single
element, return it without calling FN. If LIST is empty, return
the result of calling FN with no arguments.
This function is like ‘-reduce’ (*note -reduce::) but the operation
associates from the right rather than left. In other words, it
starts from the end of LIST and flips the arguments to FN.
Conceptually, it is like replacing the conses in LIST with
applications of FN, ignoring its last link, and evaluating the
resulting expression.
This function’s anaphoric counterpart is ‘--reduce-r’. For other
folds, see also ‘-reduce-r-from’ (*note -reduce-r-from::) and
‘-reduce’ (*note -reduce::).
(-reduce-r #'- '(1 2 3 4))
⇒ -2
(-reduce-r #'list '(1 2 3 4))
⇒ '(1 (2 (3 4)))
(--reduce-r (format "%s-%d" acc it) '(1 2 3))
⇒ "3-2-1"
-- Function: -reductions-from (fn init list)
Return a list of FN’s intermediate reductions across LIST. That
is, a list of the intermediate values of the accumulator when
‘-reduce-from’ (*note -reduce-from::) (which see) is called with
the same arguments. This function’s anaphoric counterpart is
‘--reductions-from’. For other folds, see also ‘-reductions’
(*note -reductions::) and ‘-reductions-r’ (*note -reductions-r::).
(-reductions-from #'max 0 '(2 1 4 3))
⇒ '(0 2 2 4 4)
(-reductions-from #'* 1 '(1 2 3 4))
⇒ '(1 1 2 6 24)
(--reductions-from (format "(FN %s %d)" acc it) "INIT" '(1 2 3))
⇒ '("INIT" "(FN INIT 1)" "(FN (FN INIT 1) 2)" "(FN (FN (FN INIT 1) 2) 3)")
-- Function: -reductions-r-from (fn init list)
Return a list of FN’s intermediate reductions across reversed LIST.
That is, a list of the intermediate values of the accumulator when
‘-reduce-r-from’ (*note -reduce-r-from::) (which see) is called
with the same arguments. This function’s anaphoric counterpart is
‘--reductions-r-from’. For other folds, see also ‘-reductions’
(*note -reductions::) and ‘-reductions-r’ (*note -reductions-r::).
(-reductions-r-from #'max 0 '(2 1 4 3))
⇒ '(4 4 4 3 0)
(-reductions-r-from #'* 1 '(1 2 3 4))
⇒ '(24 24 12 4 1)
(--reductions-r-from (format "(FN %d %s)" it acc) "INIT" '(1 2 3))
⇒ '("(FN 1 (FN 2 (FN 3 INIT)))" "(FN 2 (FN 3 INIT))" "(FN 3 INIT)" "INIT")
-- Function: -reductions (fn list)
Return a list of FN’s intermediate reductions across LIST. That
is, a list of the intermediate values of the accumulator when
‘-reduce’ (*note -reduce::) (which see) is called with the same
arguments. This function’s anaphoric counterpart is
‘--reductions’. For other folds, see also ‘-reductions’ (*note
-reductions::) and ‘-reductions-r’ (*note -reductions-r::).
(-reductions #'+ '(1 2 3 4))
⇒ '(1 3 6 10)
(-reductions #'* '(1 2 3 4))
⇒ '(1 2 6 24)
(--reductions (format "(FN %s %d)" acc it) '(1 2 3))
⇒ '(1 "(FN 1 2)" "(FN (FN 1 2) 3)")
-- Function: -reductions-r (fn list)
Return a list of FN’s intermediate reductions across reversed LIST.
That is, a list of the intermediate values of the accumulator when
‘-reduce-r’ (*note -reduce-r::) (which see) is called with the same
arguments. This function’s anaphoric counterpart is
‘--reductions-r’. For other folds, see also ‘-reductions-r-from’
(*note -reductions-r-from::) and ‘-reductions’ (*note
-reductions::).
(-reductions-r #'+ '(1 2 3 4))
⇒ '(10 9 7 4)
(-reductions-r #'* '(1 2 3 4))
⇒ '(24 24 12 4)
(--reductions-r (format "(FN %d %s)" it acc) '(1 2 3))
⇒ '("(FN 1 (FN 2 3))" "(FN 2 3)" 3)
-- Function: -count (pred list)
Counts the number of items in LIST where (PRED item) is non-nil.
(-count 'even? '(1 2 3 4 5))
⇒ 2
(--count (< it 4) '(1 2 3 4))
⇒ 3
-- Function: -sum (list)
Return the sum of LIST.
(-sum '())
⇒ 0
(-sum '(1))
⇒ 1
(-sum '(1 2 3 4))
⇒ 10
-- Function: -running-sum (list)
Return a list with running sums of items in LIST. LIST must be
non-empty.
(-running-sum '(1 2 3 4))
⇒ '(1 3 6 10)
(-running-sum '(1))
⇒ '(1)
(-running-sum nil)
error→ "Wrong type argument: consp, nil"
-- Function: -product (list)
Return the product of LIST.
(-product '())
⇒ 1
(-product '(1))
⇒ 1
(-product '(1 2 3 4))
⇒ 24
-- Function: -running-product (list)
Return a list with running products of items in LIST. LIST must be
non-empty.
(-running-product '(1 2 3 4))
⇒ '(1 2 6 24)
(-running-product '(1))
⇒ '(1)
(-running-product nil)
error→ "Wrong type argument: consp, nil"
-- Function: -inits (list)
Return all prefixes of LIST.
(-inits '(1 2 3 4))
⇒ '(nil (1) (1 2) (1 2 3) (1 2 3 4))
(-inits nil)
⇒ '(nil)
(-inits '(1))
⇒ '(nil (1))
-- Function: -tails (list)
Return all suffixes of LIST
(-tails '(1 2 3 4))
⇒ '((1 2 3 4) (2 3 4) (3 4) (4) nil)
(-tails nil)
⇒ '(nil)
(-tails '(1))
⇒ '((1) nil)
-- Function: -common-prefix (&rest lists)
Return the longest common prefix of LISTS.
(-common-prefix '(1))
⇒ '(1)
(-common-prefix '(1 2) '(3 4) '(1 2))
⇒ nil
(-common-prefix '(1 2) '(1 2 3) '(1 2 3 4))
⇒ '(1 2)
-- Function: -common-suffix (&rest lists)
Return the longest common suffix of LISTS.
(-common-suffix '(1))
⇒ '(1)
(-common-suffix '(1 2) '(3 4) '(1 2))
⇒ nil
(-common-suffix '(1 2 3 4) '(2 3 4) '(3 4))
⇒ '(3 4)
-- Function: -min (list)
Return the smallest value from LIST of numbers or markers.
(-min '(0))
⇒ 0
(-min '(3 2 1))
⇒ 1
(-min '(1 2 3))
⇒ 1
-- Function: -min-by (comparator list)
Take a comparison function COMPARATOR and a LIST and return the
least element of the list by the comparison function.
See also combinator ‘-on’ (*note -on::) which can transform the
values before comparing them.
(-min-by '> '(4 3 6 1))
⇒ 1
(--min-by (> (car it) (car other)) '((1 2 3) (2) (3 2)))
⇒ '(1 2 3)
(--min-by (> (length it) (length other)) '((1 2 3) (2) (3 2)))
⇒ '(2)
-- Function: -max (list)
Return the largest value from LIST of numbers or markers.
(-max '(0))
⇒ 0
(-max '(3 2 1))
⇒ 3
(-max '(1 2 3))
⇒ 3
-- Function: -max-by (comparator list)
Take a comparison function COMPARATOR and a LIST and return the
greatest element of the list by the comparison function.
See also combinator ‘-on’ (*note -on::) which can transform the
values before comparing them.
(-max-by '> '(4 3 6 1))
⇒ 6
(--max-by (> (car it) (car other)) '((1 2 3) (2) (3 2)))
⇒ '(3 2)
(--max-by (> (length it) (length other)) '((1 2 3) (2) (3 2)))
⇒ '(1 2 3)

File: dash.info, Node: Unfolding, Next: Predicates, Prev: Reductions, Up: Functions
2.5 Unfolding
=============
Operations dual to reductions, building lists from a seed value rather
than consuming a list to produce a single value.
-- Function: -iterate (fun init n)
Return a list of iterated applications of FUN to INIT.
This means a list of the form:
(INIT (FUN INIT) (FUN (FUN INIT)) ...)
N is the length of the returned list.
(-iterate #'1+ 1 10)
⇒ '(1 2 3 4 5 6 7 8 9 10)
(-iterate (lambda (x) (+ x x)) 2 5)
⇒ '(2 4 8 16 32)
(--iterate (* it it) 2 5)
⇒ '(2 4 16 256 65536)
-- Function: -unfold (fun seed)
Build a list from SEED using FUN.
This is "dual" operation to ‘-reduce-r’ (*note -reduce-r::): while
-reduce-r consumes a list to produce a single value, ‘-unfold’
(*note -unfold::) takes a seed value and builds a (potentially
infinite!) list.
FUN should return ‘nil’ to stop the generating process, or a cons
(A . B), where A will be prepended to the result and B is the new
seed.
(-unfold (lambda (x) (unless (= x 0) (cons x (1- x)))) 10)
⇒ '(10 9 8 7 6 5 4 3 2 1)
(--unfold (when it (cons it (cdr it))) '(1 2 3 4))
⇒ '((1 2 3 4) (2 3 4) (3 4) (4))
(--unfold (when it (cons it (butlast it))) '(1 2 3 4))
⇒ '((1 2 3 4) (1 2 3) (1 2) (1))

File: dash.info, Node: Predicates, Next: Partitioning, Prev: Unfolding, Up: Functions
2.6 Predicates
==============
-- Function: -any? (pred list)
Return t if (PRED x) is non-nil for any x in LIST, else nil.
Alias: ‘-any-p’, ‘-some?’, ‘-some-p’
(-any? 'even? '(1 2 3))
⇒ t
(-any? 'even? '(1 3 5))
⇒ nil
(-any? 'null '(1 3 5))
⇒ nil
-- Function: -all? (pred list)
Return t if (PRED x) is non-nil for all x in LIST, else nil.
Alias: ‘-all-p’, ‘-every?’, ‘-every-p’
(-all? 'even? '(1 2 3))
⇒ nil
(-all? 'even? '(2 4 6))
⇒ t
(--all? (= 0 (% it 2)) '(2 4 6))
⇒ t
-- Function: -none? (pred list)
Return t if (PRED x) is nil for all x in LIST, else nil.
Alias: ‘-none-p’
(-none? 'even? '(1 2 3))
⇒ nil
(-none? 'even? '(1 3 5))
⇒ t
(--none? (= 0 (% it 2)) '(1 2 3))
⇒ nil
-- Function: -only-some? (pred list)
Return ‘t‘ if at least one item of LIST matches PRED and at least
one item of LIST does not match PRED. Return ‘nil‘ both if all
items match the predicate or if none of the items match the
predicate.
Alias: ‘-only-some-p’
(-only-some? 'even? '(1 2 3))
⇒ t
(-only-some? 'even? '(1 3 5))
⇒ nil
(-only-some? 'even? '(2 4 6))
⇒ nil
-- Function: -contains? (list element)
Return non-nil if LIST contains ELEMENT.
The test for equality is done with ‘equal’, or with ‘-compare-fn’
if that’s non-nil.
Alias: ‘-contains-p’
(-contains? '(1 2 3) 1)
⇒ t
(-contains? '(1 2 3) 2)
⇒ t
(-contains? '(1 2 3) 4)
⇒ nil
-- Function: -same-items? (list list2)
Return true if LIST and LIST2 has the same items.
The order of the elements in the lists does not matter.
Alias: ‘-same-items-p’
(-same-items? '(1 2 3) '(1 2 3))
⇒ t
(-same-items? '(1 2 3) '(3 2 1))
⇒ t
(-same-items? '(1 2 3) '(1 2 3 4))
⇒ nil
-- Function: -is-prefix? (prefix list)
Return non-nil if PREFIX is prefix of LIST.
Alias: ‘-is-prefix-p’
(-is-prefix? '(1 2 3) '(1 2 3 4 5))
⇒ t
(-is-prefix? '(1 2 3 4 5) '(1 2 3))
⇒ nil
(-is-prefix? '(1 3) '(1 2 3 4 5))
⇒ nil
-- Function: -is-suffix? (suffix list)
Return non-nil if SUFFIX is suffix of LIST.
Alias: ‘-is-suffix-p’
(-is-suffix? '(3 4 5) '(1 2 3 4 5))
⇒ t
(-is-suffix? '(1 2 3 4 5) '(3 4 5))
⇒ nil
(-is-suffix? '(3 5) '(1 2 3 4 5))
⇒ nil
-- Function: -is-infix? (infix list)
Return non-nil if INFIX is infix of LIST.
This operation runs in O(n^2) time
Alias: ‘-is-infix-p’
(-is-infix? '(1 2 3) '(1 2 3 4 5))
⇒ t
(-is-infix? '(2 3 4) '(1 2 3 4 5))
⇒ t
(-is-infix? '(3 4 5) '(1 2 3 4 5))
⇒ t
-- Function: -cons-pair? (obj)
Return non-nil if OBJ is a true cons pair. That is, a cons (A .
B) where B is not a list. Alias: ‘-cons-pair-p’.
(-cons-pair? '(1 . 2))
⇒ t
(-cons-pair? '(1 2))
⇒ nil
(-cons-pair? '(1))
⇒ nil

File: dash.info, Node: Partitioning, Next: Indexing, Prev: Predicates, Up: Functions
2.7 Partitioning
================
Functions partitioning the input list into a list of lists.
-- Function: -split-at (n list)
Split LIST into two sublists after the Nth element. The result is
a list of two elements (TAKE DROP) where TAKE is a new list of the
first N elements of LIST, and DROP is the remaining elements of
LIST (not a copy). TAKE and DROP are like the results of ‘-take’
(*note -take::) and ‘-drop’ (*note -drop::), respectively, but the
split is done in a single list traversal.
(-split-at 3 '(1 2 3 4 5))
⇒ '((1 2 3) (4 5))
(-split-at 17 '(1 2 3 4 5))
⇒ '((1 2 3 4 5) nil)
(-split-at 0 '(1 2 3 4 5))
⇒ '(nil (1 2 3 4 5))
-- Function: -split-with (pred list)
Return a list of ((-take-while PRED LIST) (-drop-while PRED LIST)),
in no more than one pass through the list.
(-split-with 'even? '(1 2 3 4))
⇒ '(nil (1 2 3 4))
(-split-with 'even? '(2 4 5 6))
⇒ '((2 4) (5 6))
(--split-with (< it 4) '(1 2 3 4 3 2 1))
⇒ '((1 2 3) (4 3 2 1))
-- Macro: -split-on (item list)
Split the LIST each time ITEM is found.
Unlike ‘-partition-by’ (*note -partition-by::), the ITEM is
discarded from the results. Empty lists are also removed from the
result.
Comparison is done by ‘equal’.
See also ‘-split-when’ (*note -split-when::)
(-split-on '| '(Nil | Leaf a | Node [Tree a]))
⇒ '((Nil) (Leaf a) (Node [Tree a]))
(-split-on ':endgroup '("a" "b" :endgroup "c" :endgroup "d" "e"))
⇒ '(("a" "b") ("c") ("d" "e"))
(-split-on ':endgroup '("a" "b" :endgroup :endgroup "d" "e"))
⇒ '(("a" "b") ("d" "e"))
-- Function: -split-when (fn list)
Split the LIST on each element where FN returns non-nil.
Unlike ‘-partition-by’ (*note -partition-by::), the "matched"
element is discarded from the results. Empty lists are also
removed from the result.
This function can be thought of as a generalization of
‘split-string’.
(-split-when 'even? '(1 2 3 4 5 6))
⇒ '((1) (3) (5))
(-split-when 'even? '(1 2 3 4 6 8 9))
⇒ '((1) (3) (9))
(--split-when (memq it '(&optional &rest)) '(a b &optional c d &rest args))
⇒ '((a b) (c d) (args))
-- Function: -separate (pred list)
Return a list of ((-filter PRED LIST) (-remove PRED LIST)), in one
pass through the list.
(-separate (lambda (num) (= 0 (% num 2))) '(1 2 3 4 5 6 7))
⇒ '((2 4 6) (1 3 5 7))
(--separate (< it 5) '(3 7 5 9 3 2 1 4 6))
⇒ '((3 3 2 1 4) (7 5 9 6))
(-separate 'cdr '((1 2) (1) (1 2 3) (4)))
⇒ '(((1 2) (1 2 3)) ((1) (4)))
-- Function: -partition (n list)
Return a new list with the items in LIST grouped into N-sized
sublists. If there are not enough items to make the last group
N-sized, those items are discarded.
(-partition 2 '(1 2 3 4 5 6))
⇒ '((1 2) (3 4) (5 6))
(-partition 2 '(1 2 3 4 5 6 7))
⇒ '((1 2) (3 4) (5 6))
(-partition 3 '(1 2 3 4 5 6 7))
⇒ '((1 2 3) (4 5 6))
-- Function: -partition-all (n list)
Return a new list with the items in LIST grouped into N-sized
sublists. The last group may contain less than N items.
(-partition-all 2 '(1 2 3 4 5 6))
⇒ '((1 2) (3 4) (5 6))
(-partition-all 2 '(1 2 3 4 5 6 7))
⇒ '((1 2) (3 4) (5 6) (7))
(-partition-all 3 '(1 2 3 4 5 6 7))
⇒ '((1 2 3) (4 5 6) (7))
-- Function: -partition-in-steps (n step list)
Return a new list with the items in LIST grouped into N-sized
sublists at offsets STEP apart. If there are not enough items to
make the last group N-sized, those items are discarded.
(-partition-in-steps 2 1 '(1 2 3 4))
⇒ '((1 2) (2 3) (3 4))
(-partition-in-steps 3 2 '(1 2 3 4))
⇒ '((1 2 3))
(-partition-in-steps 3 2 '(1 2 3 4 5))
⇒ '((1 2 3) (3 4 5))
-- Function: -partition-all-in-steps (n step list)
Return a new list with the items in LIST grouped into N-sized
sublists at offsets STEP apart. The last groups may contain less
than N items.
(-partition-all-in-steps 2 1 '(1 2 3 4))
⇒ '((1 2) (2 3) (3 4) (4))
(-partition-all-in-steps 3 2 '(1 2 3 4))
⇒ '((1 2 3) (3 4))
(-partition-all-in-steps 3 2 '(1 2 3 4 5))
⇒ '((1 2 3) (3 4 5) (5))
-- Function: -partition-by (fn list)
Apply FN to each item in LIST, splitting it each time FN returns a
new value.
(-partition-by 'even? '())
⇒ '()
(-partition-by 'even? '(1 1 2 2 2 3 4 6 8))
⇒ '((1 1) (2 2 2) (3) (4 6 8))
(--partition-by (< it 3) '(1 2 3 4 3 2 1))
⇒ '((1 2) (3 4 3) (2 1))
-- Function: -partition-by-header (fn list)
Apply FN to the first item in LIST. That is the header value.
Apply FN to each item in LIST, splitting it each time FN returns
the header value, but only after seeing at least one other value
(the body).
(--partition-by-header (= it 1) '(1 2 3 1 2 1 2 3 4))
⇒ '((1 2 3) (1 2) (1 2 3 4))
(--partition-by-header (> it 0) '(1 2 0 1 0 1 2 3 0))
⇒ '((1 2 0) (1 0) (1 2 3 0))
(-partition-by-header 'even? '(2 1 1 1 4 1 3 5 6 6 1))
⇒ '((2 1 1 1) (4 1 3 5) (6 6 1))
-- Function: -partition-after-pred (pred list)
Partition directly after each time PRED is true on an element of
LIST.
(-partition-after-pred #'odd? '())
⇒ '()
(-partition-after-pred #'odd? '(1))
⇒ '((1))
(-partition-after-pred #'odd? '(0 1))
⇒ '((0 1))
-- Function: -partition-before-pred (pred list)
Partition directly before each time PRED is true on an element of
LIST.
(-partition-before-pred #'odd? '())
⇒ '()
(-partition-before-pred #'odd? '(1))
⇒ '((1))
(-partition-before-pred #'odd? '(0 1))
⇒ '((0) (1))
-- Function: -partition-before-item (item list)
Partition directly before each time ITEM appears in LIST.
(-partition-before-item 3 '())
⇒ '()
(-partition-before-item 3 '(1))
⇒ '((1))
(-partition-before-item 3 '(3))
⇒ '((3))
-- Function: -partition-after-item (item list)
Partition directly after each time ITEM appears in LIST.
(-partition-after-item 3 '())
⇒ '()
(-partition-after-item 3 '(1))
⇒ '((1))
(-partition-after-item 3 '(3))
⇒ '((3))
-- Function: -group-by (fn list)
Separate LIST into an alist whose keys are FN applied to the
elements of LIST. Keys are compared by ‘equal’.
(-group-by 'even? '())
⇒ '()
(-group-by 'even? '(1 1 2 2 2 3 4 6 8))
⇒ '((nil 1 1 3) (t 2 2 2 4 6 8))
(--group-by (car (split-string it "/")) '("a/b" "c/d" "a/e"))
⇒ '(("a" "a/b" "a/e") ("c" "c/d"))

File: dash.info, Node: Indexing, Next: Set operations, Prev: Partitioning, Up: Functions
2.8 Indexing
============
Return indices of elements based on predicates, sort elements by indices
etc.
-- Function: -elem-index (elem list)
Return the index of the first element in the given LIST which is
equal to the query element ELEM, or nil if there is no such
element.
(-elem-index 2 '(6 7 8 2 3 4))
⇒ 3
(-elem-index "bar" '("foo" "bar" "baz"))
⇒ 1
(-elem-index '(1 2) '((3) (5 6) (1 2) nil))
⇒ 2
-- Function: -elem-indices (elem list)
Return the indices of all elements in LIST equal to the query
element ELEM, in ascending order.
(-elem-indices 2 '(6 7 8 2 3 4 2 1))
⇒ '(3 6)
(-elem-indices "bar" '("foo" "bar" "baz"))
⇒ '(1)
(-elem-indices '(1 2) '((3) (1 2) (5 6) (1 2) nil))
⇒ '(1 3)
-- Function: -find-index (pred list)
Take a predicate PRED and a LIST and return the index of the first
element in the list satisfying the predicate, or nil if there is no
such element.
See also ‘-first’ (*note -first::).
(-find-index 'even? '(2 4 1 6 3 3 5 8))
⇒ 0
(--find-index (< 5 it) '(2 4 1 6 3 3 5 8))
⇒ 3
(-find-index (-partial 'string-lessp "baz") '("bar" "foo" "baz"))
⇒ 1
-- Function: -find-last-index (pred list)
Take a predicate PRED and a LIST and return the index of the last
element in the list satisfying the predicate, or nil if there is no
such element.
See also ‘-last’ (*note -last::).
(-find-last-index 'even? '(2 4 1 6 3 3 5 8))
⇒ 7
(--find-last-index (< 5 it) '(2 7 1 6 3 8 5 2))
⇒ 5
(-find-last-index (-partial 'string-lessp "baz") '("q" "foo" "baz"))
⇒ 1
-- Function: -find-indices (pred list)
Return the indices of all elements in LIST satisfying the predicate
PRED, in ascending order.
(-find-indices 'even? '(2 4 1 6 3 3 5 8))
⇒ '(0 1 3 7)
(--find-indices (< 5 it) '(2 4 1 6 3 3 5 8))
⇒ '(3 7)
(-find-indices (-partial 'string-lessp "baz") '("bar" "foo" "baz"))
⇒ '(1)
-- Function: -grade-up (comparator list)
Grade elements of LIST using COMPARATOR relation. This yields a
permutation vector such that applying this permutation to LIST
sorts it in ascending order.
(-grade-up #'< '(3 1 4 2 1 3 3))
⇒ '(1 4 3 0 5 6 2)
(let ((l '(3 1 4 2 1 3 3))) (-select-by-indices (-grade-up #'< l) l))
⇒ '(1 1 2 3 3 3 4)
-- Function: -grade-down (comparator list)
Grade elements of LIST using COMPARATOR relation. This yields a
permutation vector such that applying this permutation to LIST
sorts it in descending order.
(-grade-down #'< '(3 1 4 2 1 3 3))
⇒ '(2 0 5 6 3 1 4)
(let ((l '(3 1 4 2 1 3 3))) (-select-by-indices (-grade-down #'< l) l))
⇒ '(4 3 3 3 2 1 1)

File: dash.info, Node: Set operations, Next: Other list operations, Prev: Indexing, Up: Functions
2.9 Set operations
==================
Operations pretending lists are sets.
-- Function: -union (list list2)
Return a new list containing the elements of LIST and elements of
LIST2 that are not in LIST. The test for equality is done with
‘equal’, or with ‘-compare-fn’ if that’s non-nil.
(-union '(1 2 3) '(3 4 5))
⇒ '(1 2 3 4 5)
(-union '(1 2 3 4) '())
⇒ '(1 2 3 4)
(-union '(1 1 2 2) '(3 2 1))
⇒ '(1 1 2 2 3)
-- Function: -difference (list list2)
Return a new list with only the members of LIST that are not in
LIST2. The test for equality is done with ‘equal’, or with
‘-compare-fn’ if that’s non-nil.
(-difference '() '())
⇒ '()
(-difference '(1 2 3) '(4 5 6))
⇒ '(1 2 3)
(-difference '(1 2 3 4) '(3 4 5 6))
⇒ '(1 2)
-- Function: -intersection (list list2)
Return a new list containing only the elements that are members of
both LIST and LIST2. The test for equality is done with ‘equal’,
or with ‘-compare-fn’ if that’s non-nil.
(-intersection '() '())
⇒ '()
(-intersection '(1 2 3) '(4 5 6))
⇒ '()
(-intersection '(1 2 3 4) '(3 4 5 6))
⇒ '(3 4)
-- Function: -powerset (list)
Return the power set of LIST.
(-powerset '())
⇒ '(nil)
(-powerset '(x y z))
⇒ '((x y z) (x y) (x z) (x) (y z) (y) (z) nil)
-- Function: -permutations (list)
Return the permutations of LIST.
(-permutations '())
⇒ '(nil)
(-permutations '(1 2))
⇒ '((1 2) (2 1))
(-permutations '(a b c))
⇒ '((a b c) (a c b) (b a c) (b c a) (c a b) (c b a))
-- Function: -distinct (list)
Return a new list with all duplicates removed. The test for
equality is done with ‘equal’, or with ‘-compare-fn’ if that’s
non-nil.
Alias: ‘-uniq’
(-distinct '())
⇒ '()
(-distinct '(1 2 2 4))
⇒ '(1 2 4)
(-distinct '(t t t))
⇒ '(t)

File: dash.info, Node: Other list operations, Next: Tree operations, Prev: Set operations, Up: Functions
2.10 Other list operations
==========================
Other list functions not fit to be classified elsewhere.
-- Function: -rotate (n list)
Rotate LIST N places to the right. With N negative, rotate to the
left. The time complexity is O(n).
(-rotate 3 '(1 2 3 4 5 6 7))
⇒ '(5 6 7 1 2 3 4)
(-rotate -3 '(1 2 3 4 5 6 7))
⇒ '(4 5 6 7 1 2 3)
(-rotate 16 '(1 2 3 4 5 6 7))
⇒ '(6 7 1 2 3 4 5)
-- Function: -repeat (n x)
Return a new list of length N with each element being X. Return
nil if N is less than 1.
(-repeat 3 :a)
⇒ '(:a :a :a)
(-repeat 1 :a)
⇒ '(:a)
(-repeat 0 :a)
⇒ nil
-- Function: -cons* (&rest args)
Make a new list from the elements of ARGS. The last 2 elements of
ARGS are used as the final cons of the result, so if the final
element of ARGS is not a list, the result is a dotted list. With
no ARGS, return nil.
(-cons* 1 2)
⇒ '(1 . 2)
(-cons* 1 2 3)
⇒ '(1 2 . 3)
(-cons* 1)
⇒ 1
-- Function: -snoc (list elem &rest elements)
Append ELEM to the end of the list.
This is like ‘cons’, but operates on the end of list.
If ELEMENTS is non nil, append these to the list as well.
(-snoc '(1 2 3) 4)
⇒ '(1 2 3 4)
(-snoc '(1 2 3) 4 5 6)
⇒ '(1 2 3 4 5 6)
(-snoc '(1 2 3) '(4 5 6))
⇒ '(1 2 3 (4 5 6))
-- Function: -interpose (sep list)
Return a new list of all elements in LIST separated by SEP.
(-interpose "-" '())
⇒ '()
(-interpose "-" '("a"))
⇒ '("a")
(-interpose "-" '("a" "b" "c"))
⇒ '("a" "-" "b" "-" "c")
-- Function: -interleave (&rest lists)
Return a new list of the first item in each list, then the second
etc.
(-interleave '(1 2) '("a" "b"))
⇒ '(1 "a" 2 "b")
(-interleave '(1 2) '("a" "b") '("A" "B"))
⇒ '(1 "a" "A" 2 "b" "B")
(-interleave '(1 2 3) '("a" "b"))
⇒ '(1 "a" 2 "b")
-- Function: -iota (count &optional start step)
Return a list containing COUNT numbers. Starts from START and adds
STEP each time. The default START is zero, the default STEP is 1.
This function takes its name from the corresponding primitive in
the APL language.
(-iota 6)
⇒ '(0 1 2 3 4 5)
(-iota 4 2.5 -2)
⇒ '(2.5 0.5 -1.5 -3.5)
(-iota -1)
error→ "Wrong type argument: natnump, -1"
-- Function: -zip-with (fn list1 list2)
Zip the two lists LIST1 and LIST2 using a function FN. This
function is applied pairwise taking as first argument element of
LIST1 and as second argument element of LIST2 at corresponding
position.
The anaphoric form ‘--zip-with’ binds the elements from LIST1 as
symbol ‘it’, and the elements from LIST2 as symbol ‘other’.
(-zip-with '+ '(1 2 3) '(4 5 6))
⇒ '(5 7 9)
(-zip-with 'cons '(1 2 3) '(4 5 6))
⇒ '((1 . 4) (2 . 5) (3 . 6))
(--zip-with (concat it " and " other) '("Batman" "Jekyll") '("Robin" "Hyde"))
⇒ '("Batman and Robin" "Jekyll and Hyde")
-- Function: -zip (&rest lists)
Zip LISTS together. Group the head of each list, followed by the
second elements of each list, and so on. The lengths of the
returned groupings are equal to the length of the shortest input
list.
If two lists are provided as arguments, return the groupings as a
list of cons cells. Otherwise, return the groupings as a list of
lists.
Use ‘-zip-lists’ (*note -zip-lists::) if you need the return value
to always be a list of lists.
Alias: ‘-zip-pair’
See also: ‘-zip-lists’ (*note -zip-lists::)
(-zip '(1 2 3) '(4 5 6))
⇒ '((1 . 4) (2 . 5) (3 . 6))
(-zip '(1 2 3) '(4 5 6 7))
⇒ '((1 . 4) (2 . 5) (3 . 6))
(-zip '(1 2) '(3 4 5) '(6))
⇒ '((1 3 6))
-- Function: -zip-lists (&rest lists)
Zip LISTS together. Group the head of each list, followed by the
second elements of each list, and so on. The lengths of the
returned groupings are equal to the length of the shortest input
list.
The return value is always list of lists, which is a difference
from ‘-zip-pair’ which returns a cons-cell in case two input lists
are provided.
See also: ‘-zip’ (*note -zip::)
(-zip-lists '(1 2 3) '(4 5 6))
⇒ '((1 4) (2 5) (3 6))
(-zip-lists '(1 2 3) '(4 5 6 7))
⇒ '((1 4) (2 5) (3 6))
(-zip-lists '(1 2) '(3 4 5) '(6))
⇒ '((1 3 6))
-- Function: -zip-fill (fill-value &rest lists)
Zip LISTS, with FILL-VALUE padded onto the shorter lists. The
lengths of the returned groupings are equal to the length of the
longest input list.
(-zip-fill 0 '(1 2 3 4 5) '(6 7 8 9))
⇒ '((1 . 6) (2 . 7) (3 . 8) (4 . 9) (5 . 0))
-- Function: -unzip (lists)
Unzip LISTS.
This works just like ‘-zip’ (*note -zip::) but takes a list of
lists instead of a variable number of arguments, such that
(-unzip (-zip L1 L2 L3 ...))
is identity (given that the lists are the same length).
Note in particular that calling this on a list of two lists will
return a list of cons-cells such that the above identity works.
See also: ‘-zip’ (*note -zip::)
(-unzip (-zip '(1 2 3) '(a b c) '("e" "f" "g")))
⇒ '((1 2 3) (a b c) ("e" "f" "g"))
(-unzip '((1 2) (3 4) (5 6) (7 8) (9 10)))
⇒ '((1 3 5 7 9) (2 4 6 8 10))
(-unzip '((1 2) (3 4)))
⇒ '((1 . 3) (2 . 4))
-- Function: -cycle (list)
Return an infinite circular copy of LIST. The returned list cycles
through the elements of LIST and repeats from the beginning.
(-take 5 (-cycle '(1 2 3)))
⇒ '(1 2 3 1 2)
(-take 7 (-cycle '(1 "and" 3)))
⇒ '(1 "and" 3 1 "and" 3 1)
(-zip (-cycle '(1 2 3)) '(1 2))
⇒ '((1 . 1) (2 . 2))
-- Function: -pad (fill-value &rest lists)
Appends FILL-VALUE to the end of each list in LISTS such that they
will all have the same length.
(-pad 0 '())
⇒ '(nil)
(-pad 0 '(1))
⇒ '((1))
(-pad 0 '(1 2 3) '(4 5))
⇒ '((1 2 3) (4 5 0))
-- Function: -table (fn &rest lists)
Compute outer product of LISTS using function FN.
The function FN should have the same arity as the number of
supplied lists.
The outer product is computed by applying fn to all possible
combinations created by taking one element from each list in order.
The dimension of the result is (length lists).
See also: ‘-table-flat’ (*note -table-flat::)
(-table '* '(1 2 3) '(1 2 3))
⇒ '((1 2 3) (2 4 6) (3 6 9))
(-table (lambda (a b) (-sum (-zip-with '* a b))) '((1 2) (3 4)) '((1 3) (2 4)))
⇒ '((7 15) (10 22))
(apply '-table 'list (-repeat 3 '(1 2)))
⇒ '((((1 1 1) (2 1 1)) ((1 2 1) (2 2 1))) (((1 1 2) (2 1 2)) ((1 2 2) (2 2 2))))
-- Function: -table-flat (fn &rest lists)
Compute flat outer product of LISTS using function FN.
The function FN should have the same arity as the number of
supplied lists.
The outer product is computed by applying fn to all possible
combinations created by taking one element from each list in order.
The results are flattened, ignoring the tensor structure of the
result. This is equivalent to calling:
(-flatten-n (1- (length lists)) (apply ’-table fn lists))
but the implementation here is much more efficient.
See also: ‘-flatten-n’ (*note -flatten-n::), ‘-table’ (*note
-table::)
(-table-flat 'list '(1 2 3) '(a b c))
⇒ '((1 a) (2 a) (3 a) (1 b) (2 b) (3 b) (1 c) (2 c) (3 c))
(-table-flat '* '(1 2 3) '(1 2 3))
⇒ '(1 2 3 2 4 6 3 6 9)
(apply '-table-flat 'list (-repeat 3 '(1 2)))
⇒ '((1 1 1) (2 1 1) (1 2 1) (2 2 1) (1 1 2) (2 1 2) (1 2 2) (2 2 2))
-- Function: -first (pred list)
Return the first item in LIST for which PRED returns non-nil.
Return nil if no such element is found. To get the first item in
the list no questions asked, use ‘car’. Alias: ‘-find’. This
function’s anaphoric counterpart is ‘--first’.
(-first #'natnump '(-1 0 1))
⇒ 0
(-first #'null '(1 2 3))
⇒ nil
(--first (> it 2) '(1 2 3))
⇒ 3
-- Function: -some (pred list)
Return (PRED x) for the first LIST item where (PRED x) is non-nil,
else nil. Alias: ‘-any’. This function’s anaphoric counterpart is
‘--some’.
(-some (lambda (s) (string-match-p "x" s)) '("foo" "axe" "xor"))
⇒ 1
(-some (lambda (s) (string-match-p "x" s)) '("foo" "bar" "baz"))
⇒ nil
(--some (member 'foo it) '((foo bar) (baz)))
⇒ '(foo bar)
-- Function: -last (pred list)
Return the last x in LIST where (PRED x) is non-nil, else nil.
(-last 'even? '(1 2 3 4 5 6 3 3 3))
⇒ 6
(-last 'even? '(1 3 7 5 9))
⇒ nil
(--last (> (length it) 3) '("a" "looong" "word" "and" "short" "one"))
⇒ "short"
-- Function: -first-item (list)
Return the first item of LIST, or nil on an empty list.
See also: ‘-second-item’ (*note -second-item::), ‘-last-item’
(*note -last-item::).
(fn LIST)
(-first-item '(1 2 3))
⇒ 1
(-first-item nil)
⇒ nil
(let ((list (list 1 2 3))) (setf (-first-item list) 5) list)
⇒ '(5 2 3)
-- Function: -second-item (arg1)
Return the second item of LIST, or nil if LIST is too short.
See also: ‘-third-item’ (*note -third-item::).
(fn LIST)
(-second-item '(1 2 3))
⇒ 2
(-second-item nil)
⇒ nil
-- Function: -third-item (arg1)
Return the third item of LIST, or nil if LIST is too short.
See also: ‘-fourth-item’ (*note -fourth-item::).
(fn LIST)
(-third-item '(1 2 3))
⇒ 3
(-third-item nil)
⇒ nil
-- Function: -fourth-item (list)
Return the fourth item of LIST, or nil if LIST is too short.
See also: ‘-fifth-item’ (*note -fifth-item::).
(-fourth-item '(1 2 3 4))
⇒ 4
(-fourth-item nil)
⇒ nil
-- Function: -fifth-item (list)
Return the fifth item of LIST, or nil if LIST is too short.
See also: ‘-last-item’ (*note -last-item::).
(-fifth-item '(1 2 3 4 5))
⇒ 5
(-fifth-item nil)
⇒ nil
-- Function: -last-item (list)
Return the last item of LIST, or nil on an empty list.
(-last-item '(1 2 3))
⇒ 3
(-last-item nil)
⇒ nil
(let ((list (list 1 2 3))) (setf (-last-item list) 5) list)
⇒ '(1 2 5)
-- Function: -butlast (list)
Return a list of all items in list except for the last.
(-butlast '(1 2 3))
⇒ '(1 2)
(-butlast '(1 2))
⇒ '(1)
(-butlast '(1))
⇒ nil
-- Function: -sort (comparator list)
Sort LIST, stably, comparing elements using COMPARATOR. Return the
sorted list. LIST is NOT modified by side effects. COMPARATOR is
called with two elements of LIST, and should return non-nil if the
first element should sort before the second.
(-sort '< '(3 1 2))
⇒ '(1 2 3)
(-sort '> '(3 1 2))
⇒ '(3 2 1)
(--sort (< it other) '(3 1 2))
⇒ '(1 2 3)
-- Function: -list (&optional arg &rest args)
Ensure ARG is a list. If ARG is already a list, return it as is
(not a copy). Otherwise, return a new list with ARG as its only
element.
Another supported calling convention is (-list &rest ARGS). In
this case, if ARG is not a list, a new list with all of ARGS as
elements is returned. This use is supported for backward
compatibility and is otherwise deprecated.
(-list 1)
⇒ '(1)
(-list nil)
⇒ nil
(-list '(1 2 3))
⇒ '(1 2 3)
-- Function: -fix (fn list)
Compute the (least) fixpoint of FN with initial input LIST.
FN is called at least once, results are compared with ‘equal’.
(-fix (lambda (l) (-non-nil (--mapcat (-split-at (/ (length it) 2) it) l))) '((1 2 3)))
⇒ '((1) (2) (3))
(let ((l '((starwars scifi) (jedi starwars warrior)))) (--fix (-uniq (--mapcat (cons it (cdr (assq it l))) it)) '(jedi book)))
⇒ '(jedi starwars warrior scifi book)

File: dash.info, Node: Tree operations, Next: Threading macros, Prev: Other list operations, Up: Functions
2.11 Tree operations
====================
Functions pretending lists are trees.
-- Function: -tree-seq (branch children tree)
Return a sequence of the nodes in TREE, in depth-first search
order.
BRANCH is a predicate of one argument that returns non-nil if the
passed argument is a branch, that is, a node that can have
children.
CHILDREN is a function of one argument that returns the children of
the passed branch node.
Non-branch nodes are simply copied.
(-tree-seq 'listp 'identity '(1 (2 3) 4 (5 (6 7))))
⇒ '((1 (2 3) 4 (5 (6 7))) 1 (2 3) 2 3 4 (5 (6 7)) 5 (6 7) 6 7)
(-tree-seq 'listp 'reverse '(1 (2 3) 4 (5 (6 7))))
⇒ '((1 (2 3) 4 (5 (6 7))) (5 (6 7)) (6 7) 7 6 5 4 (2 3) 3 2 1)
(--tree-seq (vectorp it) (append it nil) [1 [2 3] 4 [5 [6 7]]])
⇒ '([1 [2 3] 4 [5 [6 7]]] 1 [2 3] 2 3 4 [5 [6 7]] 5 [6 7] 6 7)
-- Function: -tree-map (fn tree)
Apply FN to each element of TREE while preserving the tree
structure.
(-tree-map '1+ '(1 (2 3) (4 (5 6) 7)))
⇒ '(2 (3 4) (5 (6 7) 8))
(-tree-map '(lambda (x) (cons x (expt 2 x))) '(1 (2 3) 4))
⇒ '((1 . 2) ((2 . 4) (3 . 8)) (4 . 16))
(--tree-map (length it) '("<body>" ("<p>" "text" "</p>") "</body>"))
⇒ '(6 (3 4 4) 7)
-- Function: -tree-map-nodes (pred fun tree)
Call FUN on each node of TREE that satisfies PRED.
If PRED returns nil, continue descending down this node. If PRED
returns non-nil, apply FUN to this node and do not descend further.
(-tree-map-nodes 'vectorp (lambda (x) (-sum (append x nil))) '(1 [2 3] 4 (5 [6 7] 8)))
⇒ '(1 5 4 (5 13 8))
(-tree-map-nodes 'keywordp (lambda (x) (symbol-name x)) '(1 :foo 4 ((5 6 :bar) :baz 8)))
⇒ '(1 ":foo" 4 ((5 6 ":bar") ":baz" 8))
(--tree-map-nodes (eq (car-safe it) 'add-mode) (-concat it (list :mode 'emacs-lisp-mode)) '(with-mode emacs-lisp-mode (foo bar) (add-mode a b) (baz (add-mode c d))))
⇒ '(with-mode emacs-lisp-mode (foo bar) (add-mode a b :mode emacs-lisp-mode) (baz (add-mode c d :mode emacs-lisp-mode)))
-- Function: -tree-reduce (fn tree)
Use FN to reduce elements of list TREE. If elements of TREE are
lists themselves, apply the reduction recursively.
FN is first applied to first element of the list and second
element, then on this result and third element from the list etc.
See ‘-reduce-r’ (*note -reduce-r::) for how exactly are lists of
zero or one element handled.
(-tree-reduce '+ '(1 (2 3) (4 5)))
⇒ 15
(-tree-reduce 'concat '("strings" (" on" " various") ((" levels"))))
⇒ "strings on various levels"
(--tree-reduce (cond ((stringp it) (concat it " " acc)) (t (let ((sn (symbol-name it))) (concat "<" sn ">" acc "</" sn ">")))) '(body (p "some words") (div "more" (b "bold") "words")))
⇒ "<body><p>some words</p> <div>more <b>bold</b> words</div></body>"
-- Function: -tree-reduce-from (fn init-value tree)
Use FN to reduce elements of list TREE. If elements of TREE are
lists themselves, apply the reduction recursively.
FN is first applied to INIT-VALUE and first element of the list,
then on this result and second element from the list etc.
The initial value is ignored on cons pairs as they always contain
two elements.
(-tree-reduce-from '+ 1 '(1 (1 1) ((1))))
⇒ 8
(--tree-reduce-from (-concat acc (list it)) nil '(1 (2 3 (4 5)) (6 7)))
⇒ '((7 6) ((5 4) 3 2) 1)
-- Function: -tree-mapreduce (fn folder tree)
Apply FN to each element of TREE, and make a list of the results.
If elements of TREE are lists themselves, apply FN recursively to
elements of these nested lists.
Then reduce the resulting lists using FOLDER and initial value
INIT-VALUE. See ‘-reduce-r-from’ (*note -reduce-r-from::).
This is the same as calling ‘-tree-reduce’ (*note -tree-reduce::)
after ‘-tree-map’ (*note -tree-map::) but is twice as fast as it
only traverse the structure once.
(-tree-mapreduce 'list 'append '(1 (2 (3 4) (5 6)) (7 (8 9))))
⇒ '(1 2 3 4 5 6 7 8 9)
(--tree-mapreduce 1 (+ it acc) '(1 (2 (4 9) (2 1)) (7 (4 3))))
⇒ 9
(--tree-mapreduce 0 (max acc (1+ it)) '(1 (2 (4 9) (2 1)) (7 (4 3))))
⇒ 3
-- Function: -tree-mapreduce-from (fn folder init-value tree)
Apply FN to each element of TREE, and make a list of the results.
If elements of TREE are lists themselves, apply FN recursively to
elements of these nested lists.
Then reduce the resulting lists using FOLDER and initial value
INIT-VALUE. See ‘-reduce-r-from’ (*note -reduce-r-from::).
This is the same as calling ‘-tree-reduce-from’ (*note
-tree-reduce-from::) after ‘-tree-map’ (*note -tree-map::) but is
twice as fast as it only traverse the structure once.
(-tree-mapreduce-from 'identity '* 1 '(1 (2 (3 4) (5 6)) (7 (8 9))))
⇒ 362880
(--tree-mapreduce-from (+ it it) (cons it acc) nil '(1 (2 (4 9) (2 1)) (7 (4 3))))
⇒ '(2 (4 (8 18) (4 2)) (14 (8 6)))
(concat "{" (--tree-mapreduce-from (cond ((-cons-pair? it) (concat (symbol-name (car it)) " -> " (symbol-name (cdr it)))) (t (concat (symbol-name it) " : {"))) (concat it (unless (or (equal acc "}") (equal (substring it (1- (length it))) "{")) ", ") acc) "}" '((elips-mode (foo (bar . booze)) (baz . qux)) (c-mode (foo . bla) (bum . bam)))))
⇒ "{elips-mode : {foo : {bar -> booze{, baz -> qux{, c-mode : {foo -> bla, bum -> bam}}"
-- Function: -clone (list)
Create a deep copy of LIST. The new list has the same elements and
structure but all cons are replaced with new ones. This is useful
when you need to clone a structure such as plist or alist.
(let* ((a '(1 2 3)) (b (-clone a))) (nreverse a) b)
⇒ '(1 2 3)

File: dash.info, Node: Threading macros, Next: Binding, Prev: Tree operations, Up: Functions
2.12 Threading macros
=====================
-- Macro: -> (x &optional form &rest more)
Thread the expr through the forms. Insert X as the second item in
the first form, making a list of it if it is not a list already.
If there are more forms, insert the first form as the second item
in second form, etc.
(-> '(2 3 5))
⇒ '(2 3 5)
(-> '(2 3 5) (append '(8 13)))
⇒ '(2 3 5 8 13)
(-> '(2 3 5) (append '(8 13)) (-slice 1 -1))
⇒ '(3 5 8)
-- Macro: ->> (x &optional form &rest more)
Thread the expr through the forms. Insert X as the last item in
the first form, making a list of it if it is not a list already.
If there are more forms, insert the first form as the last item in
second form, etc.
(->> '(1 2 3) (-map 'square))
⇒ '(1 4 9)
(->> '(1 2 3) (-map 'square) (-remove 'even?))
⇒ '(1 9)
(->> '(1 2 3) (-map 'square) (-reduce '+))
⇒ 14
-- Macro: --> (x &rest forms)
Starting with the value of X, thread each expression through FORMS.
Insert X at the position signified by the symbol ‘it’ in the first
form. If there are more forms, insert the first form at the
position signified by ‘it’ in in second form, etc.
(--> "def" (concat "abc" it "ghi"))
⇒ "abcdefghi"
(--> "def" (concat "abc" it "ghi") (upcase it))
⇒ "ABCDEFGHI"
(--> "def" (concat "abc" it "ghi") upcase)
⇒ "ABCDEFGHI"
-- Macro: -as-> (value variable &rest forms)
Starting with VALUE, thread VARIABLE through FORMS.
In the first form, bind VARIABLE to VALUE. In the second form,
bind VARIABLE to the result of the first form, and so forth.
(-as-> 3 my-var (1+ my-var) (list my-var) (mapcar (lambda (ele) (* 2 ele)) my-var))
⇒ '(8)
(-as-> 3 my-var 1+)
⇒ 4
(-as-> 3 my-var)
⇒ 3
-- Macro: -some-> (x &optional form &rest more)
When expr is non-nil, thread it through the first form (via ‘->’
(*note ->::)), and when that result is non-nil, through the next
form, etc.
(-some-> '(2 3 5))
⇒ '(2 3 5)
(-some-> 5 square)
⇒ 25
(-some-> 5 even? square)
⇒ nil
-- Macro: -some->> (x &optional form &rest more)
When expr is non-nil, thread it through the first form (via ‘->>’
(*note ->>::)), and when that result is non-nil, through the next
form, etc.
(-some->> '(1 2 3) (-map 'square))
⇒ '(1 4 9)
(-some->> '(1 3 5) (-last 'even?) (+ 100))
⇒ nil
(-some->> '(2 4 6) (-last 'even?) (+ 100))
⇒ 106
-- Macro: -some--> (x &optional form &rest more)
When expr is non-nil, thread it through the first form (via ‘-->’
(*note -->::)), and when that result is non-nil, through the next
form, etc.
(-some--> "def" (concat "abc" it "ghi"))
⇒ "abcdefghi"
(-some--> nil (concat "abc" it "ghi"))
⇒ nil
(-some--> '(1 3 5) (-filter 'even? it) (append it it) (-map 'square it))
⇒ nil
-- Macro: -doto (init &rest forms)
Evaluate INIT and pass it as argument to FORMS with ‘->’ (*note
->::). The RESULT of evaluating INIT is threaded through each of
FORMS individually using ‘->’ (*note ->::), which see. The return
value is RESULT, which FORMS may have modified by side effect.
(-doto (list 1 2 3) pop pop)
⇒ '(3)
(-doto (cons 1 2) (setcar 3) (setcdr 4))
⇒ '(3 . 4)
(gethash 'k (--doto (make-hash-table) (puthash 'k 'v it)))
⇒ 'v

File: dash.info, Node: Binding, Next: Side effects, Prev: Threading macros, Up: Functions
2.13 Binding
============
Convenient versions of ‘let‘ and ‘let*‘ constructs combined with flow
control.
-- Macro: -when-let (var-val &rest body)
If VAL evaluates to non-nil, bind it to VAR and execute body.
Note: binding is done according to ‘-let’ (*note -let::).
(fn (VAR VAL) &rest BODY)
(-when-let (match-index (string-match "d" "abcd")) (+ match-index 2))
⇒ 5
(-when-let ((&plist :foo foo) (list :foo "foo")) foo)
⇒ "foo"
(-when-let ((&plist :foo foo) (list :bar "bar")) foo)
⇒ nil
-- Macro: -when-let* (vars-vals &rest body)
If all VALS evaluate to true, bind them to their corresponding VARS
and execute body. VARS-VALS should be a list of (VAR VAL) pairs.
Note: binding is done according to ‘-let*’ (*note -let*::). VALS
are evaluated sequentially, and evaluation stops after the first
nil VAL is encountered.
(-when-let* ((x 5) (y 3) (z (+ y 4))) (+ x y z))
⇒ 15
(-when-let* ((x 5) (y nil) (z 7)) (+ x y z))
⇒ nil
-- Macro: -if-let (var-val then &rest else)
If VAL evaluates to non-nil, bind it to VAR and do THEN, otherwise
do ELSE.
Note: binding is done according to ‘-let’ (*note -let::).
(fn (VAR VAL) THEN &rest ELSE)
(-if-let (match-index (string-match "d" "abc")) (+ match-index 3) 7)
⇒ 7
(--if-let (even? 4) it nil)
⇒ t
-- Macro: -if-let* (vars-vals then &rest else)
If all VALS evaluate to true, bind them to their corresponding VARS
and do THEN, otherwise do ELSE. VARS-VALS should be a list of (VAR
VAL) pairs.
Note: binding is done according to ‘-let*’ (*note -let*::). VALS
are evaluated sequentially, and evaluation stops after the first
nil VAL is encountered.
(-if-let* ((x 5) (y 3) (z 7)) (+ x y z) "foo")
⇒ 15
(-if-let* ((x 5) (y nil) (z 7)) (+ x y z) "foo")
⇒ "foo"
(-if-let* (((_ _ x) '(nil nil 7))) x)
⇒ 7
-- Macro: -let (varlist &rest body)
Bind variables according to VARLIST then eval BODY.
VARLIST is a list of lists of the form (PATTERN SOURCE). Each
PATTERN is matched against the SOURCE "structurally". SOURCE is
only evaluated once for each PATTERN. Each PATTERN is matched
recursively, and can therefore contain sub-patterns which are
matched against corresponding sub-expressions of SOURCE.
All the SOURCEs are evalled before any symbols are bound (i.e. "in
parallel").
If VARLIST only contains one (PATTERN SOURCE) element, you can
optionally specify it using a vector and discarding the outer-most
parens. Thus
(-let ((PATTERN SOURCE)) ...)
becomes
(-let [PATTERN SOURCE] ...).
‘-let’ (*note -let::) uses a convention of not binding places
(symbols) starting with _ whenever it’s possible. You can use this
to skip over entries you don’t care about. However, this is not
*always* possible (as a result of implementation) and these symbols
might get bound to undefined values.
Following is the overview of supported patterns. Remember that
patterns can be matched recursively, so every a, b, aK in the
following can be a matching construct and not necessarily a
symbol/variable.
Symbol:
a - bind the SOURCE to A. This is just like regular ‘let’.
Conses and lists:
(a) - bind ‘car’ of cons/list to A
(a . b) - bind car of cons to A and ‘cdr’ to B
(a b) - bind car of list to A and ‘cadr’ to B
(a1 a2 a3 ...) - bind 0th car of list to A1, 1st to A2, 2nd to
A3...
(a1 a2 a3 ... aN . rest) - as above, but bind the Nth cdr to REST.
Vectors:
[a] - bind 0th element of a non-list sequence to A (works with
vectors, strings, bit arrays...)
[a1 a2 a3 ...] - bind 0th element of non-list sequence to A0, 1st
to A1, 2nd to A2, ... If the PATTERN is shorter than SOURCE, the
values at places not in PATTERN are ignored. If the PATTERN is
longer than SOURCE, an ‘error’ is thrown.
[a1 a2 a3 ... &rest rest] - as above, but bind the rest of the
sequence to REST. This is conceptually the same as improper list
matching (a1 a2 ... aN . rest)
Key/value stores:
(&plist key0 a0 ... keyN aN) - bind value mapped by keyK in the
SOURCE plist to aK. If the value is not found, aK is nil. Uses
‘plist-get’ to fetch values.
(&alist key0 a0 ... keyN aN) - bind value mapped by keyK in the
SOURCE alist to aK. If the value is not found, aK is nil. Uses
‘assoc’ to fetch values.
(&hash key0 a0 ... keyN aN) - bind value mapped by keyK in the
SOURCE hash table to aK. If the value is not found, aK is nil.
Uses ‘gethash’ to fetch values.
Further, special keyword &keys supports "inline" matching of
plist-like key-value pairs, similarly to &keys keyword of
‘cl-defun’.
(a1 a2 ... aN &keys key1 b1 ... keyN bK)
This binds N values from the list to a1 ... aN, then interprets the
cdr as a plist (see key/value matching above).
A shorthand notation for kv-destructuring exists which allows the
patterns be optionally left out and derived from the key name in
the following fashion:
- a key :foo is converted into ‘foo’ pattern, - a key ’bar is
converted into ‘bar’ pattern, - a key "baz" is converted into ‘baz’
pattern.
That is, the entire value under the key is bound to the derived
variable without any further destructuring.
This is possible only when the form following the key is not a
valid pattern (i.e. not a symbol, a cons cell or a vector).
Otherwise the matching proceeds as usual and in case of an invalid
spec fails with an error.
Thus the patterns are normalized as follows:
;; derive all the missing patterns (&plist :foo ’bar "baz") =>
(&plist :foo foo ’bar bar "baz" baz)
;; we can specify some but not others (&plist :foo ’bar
explicit-bar) => (&plist :foo foo ’bar explicit-bar)
;; nothing happens, we store :foo in x (&plist :foo x) => (&plist
:foo x)
;; nothing happens, we match recursively (&plist :foo (a b c)) =>
(&plist :foo (a b c))
You can name the source using the syntax SYMBOL &as PATTERN. This
syntax works with lists (proper or improper), vectors and all types
of maps.
(list &as a b c) (list 1 2 3)
binds A to 1, B to 2, C to 3 and LIST to (1 2 3).
Similarly:
(bounds &as beg . end) (cons 1 2)
binds BEG to 1, END to 2 and BOUNDS to (1 . 2).
(items &as first . rest) (list 1 2 3)
binds FIRST to 1, REST to (2 3) and ITEMS to (1 2 3)
[vect &as _ b c] [1 2 3]
binds B to 2, C to 3 and VECT to [1 2 3] (_ avoids binding as
usual).
(plist &as &plist :b b) (list :a 1 :b 2 :c 3)
binds B to 2 and PLIST to (:a 1 :b 2 :c 3). Same for &alist and
&hash.
This is especially useful when we want to capture the result of a
computation and destructure at the same time. Consider the form
(function-returning-complex-structure) returning a list of two
vectors with two items each. We want to capture this entire result
and pass it to another computation, but at the same time we want to
get the second item from each vector. We can achieve it with
pattern
(result &as [_ a] [_ b]) (function-returning-complex-structure)
Note: Clojure programmers may know this feature as the ":as
binding". The difference is that we put the &as at the front
because we need to support improper list binding.
(-let (([a (b c) d] [1 (2 3) 4])) (list a b c d))
⇒ '(1 2 3 4)
(-let [(a b c . d) (list 1 2 3 4 5 6)] (list a b c d))
⇒ '(1 2 3 (4 5 6))
(-let [(&plist :foo foo :bar bar) (list :baz 3 :foo 1 :qux 4 :bar 2)] (list foo bar))
⇒ '(1 2)
-- Macro: -let* (varlist &rest body)
Bind variables according to VARLIST then eval BODY.
VARLIST is a list of lists of the form (PATTERN SOURCE). Each
PATTERN is matched against the SOURCE structurally. SOURCE is only
evaluated once for each PATTERN.
Each SOURCE can refer to the symbols already bound by this VARLIST.
This is useful if you want to destructure SOURCE recursively but
also want to name the intermediate structures.
See ‘-let’ (*note -let::) for the list of all possible patterns.
(-let* (((a . b) (cons 1 2)) ((c . d) (cons 3 4))) (list a b c d))
⇒ '(1 2 3 4)
(-let* (((a . b) (cons 1 (cons 2 3))) ((c . d) b)) (list a b c d))
⇒ '(1 (2 . 3) 2 3)
(-let* (((&alist "foo" foo "bar" bar) (list (cons "foo" 1) (cons "bar" (list 'a 'b 'c)))) ((a b c) bar)) (list foo a b c bar))
⇒ '(1 a b c (a b c))
-- Macro: -lambda (match-form &rest body)
Return a lambda which destructures its input as MATCH-FORM and
executes BODY.
Note that you have to enclose the MATCH-FORM in a pair of parens,
such that:
(-lambda (x) body) (-lambda (x y ...) body)
has the usual semantics of ‘lambda’. Furthermore, these get
translated into normal ‘lambda’, so there is no performance
penalty.
See ‘-let’ (*note -let::) for a description of the destructuring
mechanism.
(-map (-lambda ((x y)) (+ x y)) '((1 2) (3 4) (5 6)))
⇒ '(3 7 11)
(-map (-lambda ([x y]) (+ x y)) '([1 2] [3 4] [5 6]))
⇒ '(3 7 11)
(funcall (-lambda ((_ . a) (_ . b)) (-concat a b)) '(1 2 3) '(4 5 6))
⇒ '(2 3 5 6)
-- Macro: -setq (&rest forms)
Bind each MATCH-FORM to the value of its VAL.
MATCH-FORM destructuring is done according to the rules of ‘-let’
(*note -let::).
This macro allows you to bind multiple variables by destructuring
the value, so for example:
(-setq (a b) x (&plist :c c) plist)
expands roughly speaking to the following code
(setq a (car x) b (cadr x) c (plist-get plist :c))
Care is taken to only evaluate each VAL once so that in case of
multiple assignments it does not cause unexpected side effects.
(fn [MATCH-FORM VAL]...)
(let (a) (-setq a 1) a)
⇒ 1
(let (a b) (-setq (a b) (list 1 2)) (list a b))
⇒ '(1 2)
(let (c) (-setq (&plist :c c) (list :c "c")) c)
⇒ "c"

File: dash.info, Node: Side effects, Next: Destructive operations, Prev: Binding, Up: Functions
2.14 Side effects
=================
Functions iterating over lists for side effect only.
-- Function: -each (list fn)
Call FN on each element of LIST. Return nil; this function is
intended for side effects. Its anaphoric counterpart is ‘--each’.
For access to the current element’s index in LIST, see
‘-each-indexed’ (*note -each-indexed::).
(let (l) (-each '(1 2 3) (lambda (x) (push x l))) l)
⇒ '(3 2 1)
(let (l) (--each '(1 2 3) (push it l)) l)
⇒ '(3 2 1)
(-each '(1 2 3) #'identity)
⇒ nil
-- Function: -each-while (list pred fn)
Call FN on each ITEM in LIST, while (PRED ITEM) is non-nil. Once
an ITEM is reached for which PRED returns nil, FN is no longer
called. Return nil; this function is intended for side effects.
Its anaphoric counterpart is ‘--each-while’.
(let (l) (-each-while '(2 4 5 6) #'even? (lambda (x) (push x l))) l)
⇒ '(4 2)
(let (l) (--each-while '(1 2 3 4) (< it 3) (push it l)) l)
⇒ '(2 1)
(let ((s 0)) (--each-while '(1 3 4 5) (odd? it) (setq s (+ s it))) s)
⇒ 4
-- Function: -each-indexed (list fn)
Call FN on each index and element of LIST. For each ITEM at INDEX
in LIST, call (funcall FN INDEX ITEM). Return nil; this function
is intended for side effects. See also: ‘-map-indexed’ (*note
-map-indexed::).
(let (l) (-each-indexed '(a b c) (lambda (i x) (push (list x i) l))) l)
⇒ '((c 2) (b 1) (a 0))
(let (l) (--each-indexed '(a b c) (push (list it it-index) l)) l)
⇒ '((c 2) (b 1) (a 0))
(let (l) (--each-indexed nil (push it l)) l)
⇒ nil
-- Function: -each-r (list fn)
Call FN on each element of LIST in reversed order. Return nil;
this function is intended for side effects. Its anaphoric
counterpart is ‘--each-r’.
(let (l) (-each-r '(1 2 3) (lambda (x) (push x l))) l)
⇒ '(1 2 3)
(let (l) (--each-r '(1 2 3) (push it l)) l)
⇒ '(1 2 3)
(-each-r '(1 2 3) #'identity)
⇒ nil
-- Function: -each-r-while (list pred fn)
Call FN on each ITEM in reversed LIST, while (PRED ITEM) is
non-nil. Once an ITEM is reached for which PRED returns nil, FN is
no longer called. Return nil; this function is intended for side
effects. Its anaphoric counterpart is ‘--each-r-while’.
(let (l) (-each-r-while '(2 4 5 6) #'even? (lambda (x) (push x l))) l)
⇒ '(6)
(let (l) (--each-r-while '(1 2 3 4) (>= it 3) (push it l)) l)
⇒ '(3 4)
(let ((s 0)) (--each-r-while '(1 2 3 5) (odd? it) (setq s (+ s it))) s)
⇒ 8
-- Function: -dotimes (num fn)
Call FN NUM times, presumably for side effects. FN is called with
a single argument on successive integers running from 0, inclusive,
to NUM, exclusive. FN is not called if NUM is less than 1. This
function’s anaphoric counterpart is ‘--dotimes’.
(let (s) (-dotimes 3 (lambda (n) (push n s))) s)
⇒ '(2 1 0)
(let (s) (-dotimes 0 (lambda (n) (push n s))) s)
⇒ nil
(let (s) (--dotimes 5 (push it s)) s)
⇒ '(4 3 2 1 0)

File: dash.info, Node: Destructive operations, Next: Function combinators, Prev: Side effects, Up: Functions
2.15 Destructive operations
===========================
-- Macro: !cons (car cdr)
Destructive: Set CDR to the cons of CAR and CDR.
(let (l) (!cons 5 l) l)
⇒ '(5)
(let ((l '(3))) (!cons 5 l) l)
⇒ '(5 3)
-- Macro: !cdr (list)
Destructive: Set LIST to the cdr of LIST.
(let ((l '(3))) (!cdr l) l)
⇒ '()
(let ((l '(3 5))) (!cdr l) l)
⇒ '(5)

File: dash.info, Node: Function combinators, Prev: Destructive operations, Up: Functions
2.16 Function combinators
=========================
These combinators require Emacs 24 for its lexical scope. So they are
offered in a separate package: ‘dash-functional‘.
-- Function: -partial (fn &rest args)
Take a function FN and fewer than the normal arguments to FN, and
return a fn that takes a variable number of additional ARGS. When
called, the returned function calls FN with ARGS first and then
additional args.
(funcall (-partial '- 5) 3)
⇒ 2
(funcall (-partial '+ 5 2) 3)
⇒ 10
-- Function: -rpartial (fn &rest args)
Takes a function FN and fewer than the normal arguments to FN, and
returns a fn that takes a variable number of additional ARGS. When
called, the returned function calls FN with the additional args
first and then ARGS.
(funcall (-rpartial '- 5) 8)
⇒ 3
(funcall (-rpartial '- 5 2) 10)
⇒ 3
-- Function: -juxt (&rest fns)
Takes a list of functions and returns a fn that is the
juxtaposition of those fns. The returned fn takes a variable
number of args, and returns a list containing the result of
applying each fn to the args (left-to-right).
(funcall (-juxt '+ '-) 3 5)
⇒ '(8 -2)
(-map (-juxt 'identity 'square) '(1 2 3))
⇒ '((1 1) (2 4) (3 9))
-- Function: -compose (&rest fns)
Takes a list of functions and returns a fn that is the composition
of those fns. The returned fn takes a variable number of
arguments, and returns the result of applying each fn to the result
of applying the previous fn to the arguments (right-to-left).
(funcall (-compose 'square '+) 2 3)
⇒ (square (+ 2 3))
(funcall (-compose 'identity 'square) 3)
⇒ (square 3)
(funcall (-compose 'square 'identity) 3)
⇒ (square 3)
-- Function: -applify (fn)
Changes an n-arity function FN to a 1-arity function that expects a
list with n items as arguments
(-map (-applify '+) '((1 1 1) (1 2 3) (5 5 5)))
⇒ '(3 6 15)
(-map (-applify (lambda (a b c) `(,a (,b (,c))))) '((1 1 1) (1 2 3) (5 5 5)))
⇒ '((1 (1 (1))) (1 (2 (3))) (5 (5 (5))))
(funcall (-applify '<) '(3 6))
⇒ t
-- Function: -on (operator transformer)
Return a function of two arguments that first applies TRANSFORMER
to each of them and then applies OPERATOR on the results (in the
same order).
In types: (b -> b -> c) -> (a -> b) -> a -> a -> c
(-sort (-on '< 'length) '((1 2 3) (1) (1 2)))
⇒ '((1) (1 2) (1 2 3))
(-min-by (-on '> 'length) '((1 2 3) (4) (1 2)))
⇒ '(4)
(-min-by (-on 'string-lessp 'number-to-string) '(2 100 22))
⇒ 22
-- Function: -flip (func)
Swap the order of arguments for binary function FUNC.
In types: (a -> b -> c) -> b -> a -> c
(funcall (-flip '<) 2 1)
⇒ t
(funcall (-flip '-) 3 8)
⇒ 5
(-sort (-flip '<) '(4 3 6 1))
⇒ '(6 4 3 1)
-- Function: -const (c)
Return a function that returns C ignoring any additional arguments.
In types: a -> b -> a
(funcall (-const 2) 1 3 "foo")
⇒ 2
(-map (-const 1) '("a" "b" "c" "d"))
⇒ '(1 1 1 1)
(-sum (-map (-const 1) '("a" "b" "c" "d")))
⇒ 4
-- Macro: -cut (&rest params)
Take n-ary function and n arguments and specialize some of them.
Arguments denoted by <> will be left unspecialized.
See SRFI-26 for detailed description.
(funcall (-cut list 1 <> 3 <> 5) 2 4)
⇒ '(1 2 3 4 5)
(-map (-cut funcall <> 5) `(1+ 1- ,(lambda (x) (/ 1.0 x))))
⇒ '(6 4 0.2)
(-map (-cut <> 1 2 3) '(list vector string))
⇒ '((1 2 3) [1 2 3] "\^A\^B\^C")
-- Function: -not (pred)
Take a unary predicate PRED and return a unary predicate that
returns t if PRED returns nil and nil if PRED returns non-nil.
(funcall (-not 'even?) 5)
⇒ t
(-filter (-not (-partial '< 4)) '(1 2 3 4 5 6 7 8))
⇒ '(1 2 3 4)
-- Function: -orfn (&rest preds)
Take list of unary predicates PREDS and return a unary predicate
with argument x that returns non-nil if at least one of the PREDS
returns non-nil on x.
In types: [a -> Bool] -> a -> Bool
(-filter (-orfn 'even? (-partial (-flip '<) 5)) '(1 2 3 4 5 6 7 8 9 10))
⇒ '(1 2 3 4 6 8 10)
(funcall (-orfn 'stringp 'even?) "foo")
⇒ t
-- Function: -andfn (&rest preds)
Take list of unary predicates PREDS and return a unary predicate
with argument x that returns non-nil if all of the PREDS returns
non-nil on x.
In types: [a -> Bool] -> a -> Bool
(funcall (-andfn (-cut < <> 10) 'even?) 6)
⇒ t
(funcall (-andfn (-cut < <> 10) 'even?) 12)
⇒ nil
(-filter (-andfn (-not 'even?) (-cut >= 5 <>)) '(1 2 3 4 5 6 7 8 9 10))
⇒ '(1 3 5)
-- Function: -iteratefn (fn n)
Return a function FN composed N times with itself.
FN is a unary function. If you need to use a function of higher
arity, use ‘-applify’ (*note -applify::) first to turn it into a
unary function.
With n = 0, this acts as identity function.
In types: (a -> a) -> Int -> a -> a.
This function satisfies the following law:
(funcall (-iteratefn fn n) init) = (-last-item (-iterate fn init
(1+ n))).
(funcall (-iteratefn (lambda (x) (* x x)) 3) 2)
⇒ 256
(funcall (-iteratefn '1+ 3) 1)
⇒ 4
(funcall (-iteratefn 'cdr 3) '(1 2 3 4 5))
⇒ '(4 5)
-- Function: -fixfn (fn &optional equal-test halt-test)
Return a function that computes the (least) fixpoint of FN.
FN must be a unary function. The returned lambda takes a single
argument, X, the initial value for the fixpoint iteration. The
iteration halts when either of the following conditions is
satisfied:
1. Iteration converges to the fixpoint, with equality being tested
using EQUAL-TEST. If EQUAL-TEST is not specified, ‘equal’ is used.
For functions over the floating point numbers, it may be necessary
to provide an appropriate approximate comparison test.
2. HALT-TEST returns a non-nil value. HALT-TEST defaults to a
simple counter that returns t after ‘-fixfn-max-iterations’, to
guard against infinite iteration. Otherwise, HALT-TEST must be a
function that accepts a single argument, the current value of X,
and returns non-nil as long as iteration should continue. In this
way, a more sophisticated convergence test may be supplied by the
caller.
The return value of the lambda is either the fixpoint or, if
iteration halted before converging, a cons with car ‘halted’ and
cdr the final output from HALT-TEST.
In types: (a -> a) -> a -> a.
(funcall (-fixfn 'cos 'approx-equal) 0.7)
⇒ 0.7390851332151607
(funcall (-fixfn (lambda (x) (expt (+ x 10) 0.25))) 2.0)
⇒ 1.8555845286409378
(funcall (-fixfn 'sin 'approx-equal) 0.1)
⇒ '(halted . t)
-- Function: -prodfn (&rest fns)
Take a list of n functions and return a function that takes a list
of length n, applying i-th function to i-th element of the input
list. Returns a list of length n.
In types (for n=2): ((a -> b), (c -> d)) -> (a, c) -> (b, d)
This function satisfies the following laws:
(-compose (-prodfn f g ...) (-prodfn f’ g’ ...)) = (-prodfn
(-compose f f’) (-compose g g’) ...) (-prodfn f g ...) = (-juxt
(-compose f (-partial ’nth 0)) (-compose g (-partial ’nth 1)) ...)
(-compose (-prodfn f g ...) (-juxt f’ g’ ...)) = (-juxt (-compose f
f’) (-compose g g’) ...) (-compose (-partial ’nth n) (-prod f1 f2
...)) = (-compose fn (-partial ’nth n))
(funcall (-prodfn '1+ '1- 'number-to-string) '(1 2 3))
⇒ '(2 1 "3")
(-map (-prodfn '1+ '1-) '((1 2) (3 4) (5 6) (7 8)))
⇒ '((2 1) (4 3) (6 5) (8 7))
(apply '+ (funcall (-prodfn 'length 'string-to-number) '((1 2 3) "15")))
⇒ 18

File: dash.info, Node: Development, Next: FDL, Prev: Functions, Up: Top
3 Development
*************
The Dash repository is hosted on GitHub at
<https://github.com/magnars/dash.el>.
* Menu:
* Contribute:: How to contribute.
* Change log:: List of significant changes by version.
* Contributors:: List of contributors.

File: dash.info, Node: Contribute, Next: Change log, Up: Development
3.1 Contribute
==============
Yes, please do. Pure functions in the list manipulation realm only,
please. There’s a suite of examples/tests in ‘dev/examples.el’, so
remember to add tests for your additions, or they may get broken later.
Run the tests with ‘make check’. Regenerate the docs with ‘make
docs’. Contributors are encouraged to install these commands as a Git
pre-commit hook, so that the tests are always running and the docs are
always in sync:
$ cp pre-commit.sh .git/hooks/pre-commit
Oh, and don’t edit ‘README.md’ or ‘dash.texi’ directly, as they are
auto-generated. Instead, change their respective templates
‘readme-template.md’ or ‘dash-template.texi’.
To ensure that Dash can be distributed with GNU ELPA or Emacs, we
require that all contributors assign copyright to the Free Software
Foundation. For more on this, *note (emacs)Copyright Assignment::.

File: dash.info, Node: Change log, Next: Contributors, Prev: Contribute, Up: Development
3.2 Change log
==============
Changes in 2.17:
• Sped up ‘-uniq’ by using hash-tables when possible (Zhu
Zihao).
• Fixed ‘-inits’ to be non-destructive (Zach Shaftel).
• Fixed indent rules for ‘-some->’ and family (Wouter
Bolsterlee).
• Added ‘-zip-lists’ which always returns a list of proper
lists, even for two input lists (see issue #135).
Changes in 2.16:
• Added ‘--doto’, anaphoric version of ‘-doto’.
• Aliased ‘-cons-pair-p’ to ‘-cons-pair?’.
• Generalized ‘-rotate’ for |N| greater than the length of the
list (Brian Leung).
• Added a mechanism to extend destructuring with custom matchers
(Ivan Yonchovski).
Changes in 2.15:
This release brought new destructuring features, some new control
flow functions, and performance optimizations.
• Added ‘-setq’ with destructuring binding support similar to
the ‘-let’ family.
• Added smarter key destructuring in ‘-let’ and friends where
variables are auto-derived from keys.
• Allowed ‘-let’ bindings without a source value form.
• Added ‘-each-r’ and ‘-each-r-while’ (Paul Pogonyshev).
• Added ‘-common-suffix’ (Basil L. Contovounesios).
• Improved performance of folds (‘-reduce’ and friends) (Basil
L. Contovounesios).
Changes in 2.14:
This release retired support for Emacs 23.
• Added Edebug support for threading macros (Wilfred Hughes).
• Added ‘-unzip’.
• Added support for ‘-first-item’ and ‘-last-item’ as place
forms (*note (elisp)Generalized Variables::).
• Added ‘-powerset’ and ‘-permutations’ (Mark Oteiza).
• Added ‘-as->’ for threading a named variable (Zachary Kanfer).
• Added ‘-partition-after-pred’, ‘-partition-before-pred’,
‘-partition-after-item’, and ‘-partition-before-item’ (Zachary
Kanfer).
• Fixed a bug in ‘-any-p’ and friends testing for ‘null’ on
lists containing ‘nil’.
• Fixed infinite loop bug in ‘-zip’ and ‘-interleave’ when
called with empty input.
• Added ‘-second-item’ through ‘-fifth-item’ as alternatives to
‘nth’ (Wilfred Hughes).
• Added ‘-tails’ and ‘-inits’.
• Added ‘-running-sum’ and ‘-running-product’.
• Added the ‘-reductions[-r][-from]’ family of functions (like
‘-reduce’ but collecting intermediate results).
• Added ‘-common-prefix’ (Basil L. Contovounesios).
Changes in 2.13:
• ‘-let’ now supports ‘&alist’ destructuring.
• Various performance improvements.
• ‘-zip’ might change in a future release to always return a
list of proper lists. Added ‘-zip-pair’ for users who
explicitly want the old behavior.
• Enabled lexical binding in ‘dash.el’ for Emacs versions 24 or
newer.
• Added ‘-select-column’ and ‘-select-columns’.
• Fixed ‘-map-last’ and ‘--remove-last’ to be non-destructive.
• Added ‘-each-indexed’ and ‘--each-indexed’.
• Added ‘-take-last’ and ‘-drop-last’.
• Added the ‘-doto’ macro.
• ‘-cut <>’ is now treated as a function, consistent with SRFI
26 (https://srfi.schemers.org/srfi-26/srfi-26.html).
Changes in 2.12:
• Added GNU ELPA support (Phillip Lord).
• Added ‘-some->’, ‘-some->>’, and ‘-some-->’ macros (Cam Saul).
• ‘-is-suffix?’ is now non-destructive.
• Faster hash table implementation for ‘-union’.
• Improvements to docstrings and examples.
Changes in 2.11:
• Lots of clean up w.r.t. byte compilation, debug macros, and
tests.
Changes in 2.10:
• Added ‘-let’ destructuring to ‘-if-let’ and ‘-when-let’
(Fredrik Bergroth).
Changes in 2.9:
• Added ‘-let’, ‘-let*’, and ‘-lambda’ with destructuring.
• Added ‘-tree-seq’ and ‘-tree-map-nodes’.
• Added ‘-non-nil’.
• Added ‘-fix’.
• Added ‘-fixfn’ (‘dash-functional’ version 1.2).
• Added ‘-copy’ (Wilfred Hughes).
Changes in 2.8:
• Added ‘-butlast’.
Changes in 2.7:
• ‘-zip’ now supports more than two lists (Steve Lamb).
• Added ‘-cycle’, ‘-pad’, ‘-annotate’, and ‘-zip-fill’ (Steve
Lamb).
• Added ‘-table’, ‘-table-flat’ (finite Cartesian product).
• Added ‘-flatten-n’.
• ‘-slice’ now supports a “step” argument.
• Added functional combinators ‘-iteratefn’ and ‘-prodfn’.
• Added ‘-replace’, ‘-splice’, and ‘-splice-list’ which
generalize ‘-replace-at’ and ‘-insert-at’.
• Added ‘-compose’, ‘-iteratefn’, and ‘-prodfn’
(‘dash-functional’ version 1.1).
Changes in 2.6:
• Added ‘-is-prefix-p’, ‘-is-suffix-p’, and ‘-is-infix-p’ (Matus
Goljer).
• Added ‘-iterate’ and ‘-unfold’ (Matus Goljer).
• Added ‘-split-on’ and ‘-split-when’ (Matus Goljer).
• Added ‘-find-last-index’ (Matus Goljer).
• Added ‘-list’ (Johan Andersson).
Changes in 2.5:
• Added ‘-same-items?’ (Johan Andersson).
• Various bugfixes.
Changes in 2.4:
• Added ‘-snoc’ (Matus Goljer).
• Added ‘-replace-at’, ‘-update-at’, ‘-remove-at’, and
‘-remove-at-indices’ (Matus Goljer).
Changes in 2.3:
• Added tree operations (Matus Goljer).
• Made Font Lock optional.
Changes in 2.2:
• Added ‘-compose’ (Christina Whyte).
Changes in 2.1:
• Added indexing operations (Matus Goljer).
Changes in 2.0:
• Split out ‘dash-functional.el’ (Matus Goljer).
• Added ‘-andfn’, ‘-orfn’, ‘-not’, ‘-cut’, ‘-const’, ‘-flip’,
and ‘-on’ (Matus Goljer).
• Fixed ‘-min’, ‘-max’, ‘-min-by’, and ‘-max-by’ (Matus Goljer).
Changes in 1.8:
• Added ‘-first-item’ and ‘-last-item’ (Wilfred Hughes).
Changes in 1.7:
• Added ‘-rotate’ (Matus Goljer).
Changes in 1.6:
• Added ‘-min’, ‘-max’, ‘-min-by’, and ‘-max-by’ (Johan
Andersson).
Changes in 1.5:
• Added ‘-sum’ and ‘-product’ (Johan Andersson).
Changes in 1.4:
• Added ‘-sort’.
• Added ‘-reduce-r’ (Matus Goljer).
• Added ‘-reduce-r-from’ (Matus Goljer).
Changes in 1.3:
• Added ‘-partition-in-steps’.
• Added ‘-partition-all-in-steps’.
Changes in 1.2:
• Added ‘-last’ (Matus Goljer).
• Added ‘-insert-at’ (Emanuel Evans).
• Added ‘-when-let’ and ‘-if-let’ (Emanuel Evans).
• Added ‘-when-let*’ and ‘-if-let*’ (Emanuel Evans).
• Various bugfixes.

File: dash.info, Node: Contributors, Prev: Change log, Up: Development
3.3 Contributors
================
• Matus Goljer (https://github.com/Fuco1) contributed lots of
features and functions.
• Takafumi Arakaki (https://github.com/tkf) contributed ‘-group-by’.
• tali713 (https://github.com/tali713) is the author of ‘-applify’.
• Víctor M. Valenzuela (https://github.com/vemv) contributed
‘-repeat’.
• Nic Ferrier (https://github.com/nicferrier) contributed ‘-cons*’.
• Wilfred Hughes (https://github.com/Wilfred) contributed ‘-slice’,
‘-first-item’, and ‘-last-item’.
• Emanuel Evans (https://github.com/shosti) contributed ‘-if-let’,
‘-when-let’, and ‘-insert-at’.
• Johan Andersson (https://github.com/rejeep) contributed ‘-sum’,
‘-product’, and ‘-same-items?’.
• Christina Whyte (https://github.com/kurisuwhyte) contributed
‘-compose’.
• Steve Lamb (https://github.com/steventlamb) contributed ‘-cycle’,
‘-pad’, ‘-annotate’, ‘-zip-fill’, and a variadic version of ‘-zip’.
• Fredrik Bergroth (https://github.com/fbergroth) made the ‘-if-let’
family use ‘-let’ destructuring and improved the script for
generating documentation.
• Mark Oteiza (https://github.com/holomorph) contributed the script
to create an Info manual.
• Vasilij Schneidermann (https://github.com/wasamasa) contributed
‘-some’.
• William West (https://github.com/occidens) made ‘-fixfn’ more
robust at handling floats.
• Cam Saul (https://github.com/camsaul) contributed ‘-some->’,
‘-some->>’, and ‘-some-->’.
• Basil L. Contovounesios (https://github.com/basil-conto)
contributed ‘-common-prefix’.
• Paul Pogonyshev (https://github.com/doublep) contributed ‘-each-r’
and ‘-each-r-while’.
Thanks!
New contributors are very welcome. *Note Contribute::.

File: dash.info, Node: FDL, Next: GPL, Prev: Development, Up: Top
Appendix A GNU Free Documentation License
*****************************************
Version 1.3, 3 November 2008
Copyright © 2000, 2001, 2002, 2007, 2008 Free Software Foundation, Inc.
<https://fsf.org/>
Everyone is permitted to copy and distribute verbatim copies
of this license document, but changing it is not allowed.
0. PREAMBLE
The purpose of this License is to make a manual, textbook, or other
functional and useful document “free” in the sense of freedom: to
assure everyone the effective freedom to copy and redistribute it,
with or without modifying it, either commercially or
noncommercially. Secondarily, this License preserves for the
author and publisher a way to get credit for their work, while not
being considered responsible for modifications made by others.
This License is a kind of “copyleft”, which means that derivative
works of the document must themselves be free in the same sense.
It complements the GNU General Public License, which is a copyleft
license designed for free software.
We have designed this License in order to use it for manuals for
free software, because free software needs free documentation: a
free program should come with manuals providing the same freedoms
that the software does. But this License is not limited to
software manuals; it can be used for any textual work, regardless
of subject matter or whether it is published as a printed book. We
recommend this License principally for works whose purpose is
instruction or reference.
1. APPLICABILITY AND DEFINITIONS
This License applies to any manual or other work, in any medium,
that contains a notice placed by the copyright holder saying it can
be distributed under the terms of this License. Such a notice
grants a world-wide, royalty-free license, unlimited in duration,
to use that work under the conditions stated herein. The
“Document”, below, refers to any such manual or work. Any member
of the public is a licensee, and is addressed as “you”. You accept
the license if you copy, modify or distribute the work in a way
requiring permission under copyright law.
A “Modified Version” of the Document means any work containing the
Document or a portion of it, either copied verbatim, or with
modifications and/or translated into another language.
A “Secondary Section” is a named appendix or a front-matter section
of the Document that deals exclusively with the relationship of the
publishers or authors of the Document to the Document’s overall
subject (or to related matters) and contains nothing that could
fall directly within that overall subject. (Thus, if the Document
is in part a textbook of mathematics, a Secondary Section may not
explain any mathematics.) The relationship could be a matter of
historical connection with the subject or with related matters, or
of legal, commercial, philosophical, ethical or political position
regarding them.
The “Invariant Sections” are certain Secondary Sections whose
titles are designated, as being those of Invariant Sections, in the
notice that says that the Document is released under this License.
If a section does not fit the above definition of Secondary then it
is not allowed to be designated as Invariant. The Document may
contain zero Invariant Sections. If the Document does not identify
any Invariant Sections then there are none.
The “Cover Texts” are certain short passages of text that are
listed, as Front-Cover Texts or Back-Cover Texts, in the notice
that says that the Document is released under this License. A
Front-Cover Text may be at most 5 words, and a Back-Cover Text may
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A “Transparent” copy of the Document means a machine-readable copy,
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To “Preserve the Title” of such a section when you modify the
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The Document may include Warranty Disclaimers next to the notice
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You may copy and distribute the Document in any medium, either
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copyright notices, and the license notice saying this License
applies to the Document are reproduced in all copies, and that you
add no other conditions whatsoever to those of this License. You
may not use technical measures to obstruct or control the reading
or further copying of the copies you make or distribute. However,
you may accept compensation in exchange for copies. If you
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You may also lend copies, under the same conditions stated above,
and you may publicly display copies.
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It is requested, but not required, that you contact the authors of
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4. MODIFICATIONS
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to gives permission.
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unique number. Make the same adjustment to the section titles in
the list of Invariant Sections in the license notice of the
combined work.
In the combination, you must combine any sections Entitled
“History” in the various original documents, forming one section
Entitled “History”; likewise combine any sections Entitled
“Acknowledgements”, and any sections Entitled “Dedications”. You
must delete all sections Entitled “Endorsements.”
6. COLLECTIONS OF DOCUMENTS
You may make a collection consisting of the Document and other
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that is included in the collection, provided that you follow the
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You may extract a single document from such a collection, and
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License in all other respects regarding verbatim copying of that
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7. AGGREGATION WITH INDEPENDENT WORKS
A compilation of the Document or its derivatives with other
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If the Cover Text requirement of section 3 is applicable to these
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of the entire aggregate, the Document’s Cover Texts may be placed
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the whole aggregate.
8. TRANSLATION
Translation is considered a kind of modification, so you may
distribute translations of the Document under the terms of section
4. Replacing Invariant Sections with translations requires special
permission from their copyright holders, but you may include
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original versions of these Invariant Sections. You may include a
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Document, and any Warranty Disclaimers, provided that you also
include the original English version of this License and the
original versions of those notices and disclaimers. In case of a
disagreement between the translation and the original version of
this License or a notice or disclaimer, the original version will
prevail.
If a section in the Document is Entitled “Acknowledgements”,
“Dedications”, or “History”, the requirement (section 4) to
Preserve its Title (section 1) will typically require changing the
actual title.
9. TERMINATION
You may not copy, modify, sublicense, or distribute the Document
except as expressly provided under this License. Any attempt
otherwise to copy, modify, sublicense, or distribute it is void,
and will automatically terminate your rights under this License.
However, if you cease all violation of this License, then your
license from a particular copyright holder is reinstated (a)
provisionally, unless and until the copyright holder explicitly and
finally terminates your license, and (b) permanently, if the
copyright holder fails to notify you of the violation by some
reasonable means prior to 60 days after the cessation.
Moreover, your license from a particular copyright holder is
reinstated permanently if the copyright holder notifies you of the
violation by some reasonable means, this is the first time you have
received notice of violation of this License (for any work) from
that copyright holder, and you cure the violation prior to 30 days
after your receipt of the notice.
Termination of your rights under this section does not terminate
the licenses of parties who have received copies or rights from you
under this License. If your rights have been terminated and not
permanently reinstated, receipt of a copy of some or all of the
same material does not give you any rights to use it.
10. FUTURE REVISIONS OF THIS LICENSE
The Free Software Foundation may publish new, revised versions of
the GNU Free Documentation License from time to time. Such new
versions will be similar in spirit to the present version, but may
differ in detail to address new problems or concerns. See
<https://www.gnu.org/licenses/>.
Each version of the License is given a distinguishing version
number. If the Document specifies that a particular numbered
version of this License “or any later version” applies to it, you
have the option of following the terms and conditions either of
that specified version or of any later version that has been
published (not as a draft) by the Free Software Foundation. If the
Document does not specify a version number of this License, you may
choose any version ever published (not as a draft) by the Free
Software Foundation. If the Document specifies that a proxy can
decide which future versions of this License can be used, that
proxy’s public statement of acceptance of a version permanently
authorizes you to choose that version for the Document.
11. RELICENSING
“Massive Multiauthor Collaboration Site” (or “MMC Site”) means any
World Wide Web server that publishes copyrightable works and also
provides prominent facilities for anybody to edit those works. A
public wiki that anybody can edit is an example of such a server.
A “Massive Multiauthor Collaboration” (or “MMC”) contained in the
site means any set of copyrightable works thus published on the MMC
site.
“CC-BY-SA” means the Creative Commons Attribution-Share Alike 3.0
license published by Creative Commons Corporation, a not-for-profit
corporation with a principal place of business in San Francisco,
California, as well as future copyleft versions of that license
published by that same organization.
“Incorporate” means to publish or republish a Document, in whole or
in part, as part of another Document.
An MMC is “eligible for relicensing” if it is licensed under this
License, and if all works that were first published