.emacs.d/elpa/dash-20220417.2250/dash.info

This is dash.info, produced by makeinfo version 6.7 from dash.texi.

This manual is for Dash version 2.19.1.

   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 2.19.1.

   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.
* 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/), GNU-devel ELPA
(https://elpa.gnu.org/devel/), 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.

   Alternatively, you can just dump ‘dash.el’ in your ‘load-path’
somewhere (*note (emacs)Lisp Libraries::).

* 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 "2.19.1"))


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)
     Use PRED to conditionally apply REP to each item in LIST.  Return a
     copy of LIST where the items for which PRED returns ‘nil’ are
     unchanged, and the rest 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)
     Use PRED to determine the first item in LIST to call REP on.
     Return a copy of LIST where the first item for which PRED returns
     non-‘nil’ is replaced with the result of calling REP on that 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)
     Use PRED to determine the last item in LIST to call REP on.  Return
     a copy of LIST where the last item for which PRED returns non-‘nil’
     is replaced with the result of calling REP on that 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)
     Apply FN to each index and item in LIST and return the list of
     results.  This is like ‘-map’ (*note -map::), but FN takes two
     arguments: the index of the current element within LIST, and the
     element itself.

     This function’s anaphoric counterpart is ‘--map-indexed’.

     For a side-effecting variant, 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)
          (-map-indexed #'* '(1 2 3 4))
              ⇒ (0 2 6 12)

 -- 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 (list)
     Create a shallow copy of 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 is ‘--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 is ‘--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)
     Remove the last item from LIST for which PRED returns non-‘nil’.
     The result is a copy of LIST regardless of whether an element is
     removed.

     Alias: ‘-reject-last’.

     This function’s anaphoric counterpart is ‘--remove-last’.

     See also ‘-map-last’ (*note -map-last::), ‘-remove-item’ (*note
     -remove-item::), and ‘-remove-first’ (*note -remove-first::).

          (-remove-last #'natnump '(1 3 5 4 7 8 10 -11))
              ⇒ (1 3 5 4 7 8 -11)
          (-remove-last #'stringp '(1 2 "last" "second"))
              ⇒ (1 2 "last")
          (--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)
     Return a copy of LIST with all occurrences of ITEM removed.  The
     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"))
              ⇒ ("alice" "eve")

 -- Function: -non-nil (list)
     Return a copy of LIST with all ‘nil’ items removed.

          (-non-nil '(nil 1 nil 2 nil nil 3 4 nil 5 nil))
              ⇒ (1 2 3 4 5)
          (-non-nil '((nil)))
              ⇒ ((nil))
          (-non-nil ())
              ⇒ ()

 -- 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))
              ⇒ ()

 -- 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::).

          (-drop 3 '(1 2 3 4 5))
              ⇒ (4 5)
          (-drop 17 '(1 2 3 4 5))
              ⇒ ()
          (-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))
              ⇒ ()
          (-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))
              ⇒ ()
          (-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 each
     item in LIST.  Like ‘-filter’ (*note -filter::), but returns the
     non-‘nil’ results of FN instead of the corresponding elements of
     LIST.

     Its anaphoric counterpart is ‘--keep’.

          (-keep #'cdr '((1 2 3) (4 5) (6)))
              ⇒ ((2 3) (5))
          (-keep (lambda (n) (and (> n 3) (* 10 n))) '(1 2 3 4 5 6))
              ⇒ (40 50 60)
          (--keep (and (> it 3) (* 10 it)) '(1 2 3 4 5 6))
              ⇒ (40 50 60)

 -- Function: -concat (&rest sequences)
     Concatenate all the arguments and make the result a list.  The
     result is a list whose elements are the elements of all the
     arguments.  Each argument may be a list, vector or string.  The
     last argument is not copied, just used as the tail of the new list.

          (-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)
     Use FUNC to update the Nth element of LIST.  Return a copy of LIST
     where the Nth element is replaced with the result of calling FUNC
     on it.

     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 ())
              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 ())
              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))
              ⇒ ()
          (-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))
              ⇒ ()
          (-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
==============

Reductions of one or more lists to a boolean value.

 -- 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 #'stringp '(1 "2" 3))
              ⇒ t
          (--some (string-match-p "x" it) '("foo" "axe" "xor"))
              ⇒ 1
          (--some (= it-index 3) '(0 1 2))
              ⇒ nil

 -- Function: -every (pred list)
     Return non-‘nil’ if PRED returns non-‘nil’ for all items in LIST.
     If so, return the last such result of PRED.  Otherwise, once an
     item is reached for which PRED returns ‘nil’, return ‘nil’ without
     calling PRED on any further LIST elements.

     This function is like ‘-every-p’, but on success returns the last
     non-‘nil’ result of PRED instead of just ‘t’.

     This function’s anaphoric counterpart is ‘--every’.

          (-every #'numberp '(1 2 3))
              ⇒ t
          (--every (string-match-p "x" it) '("axe" "xor"))
              ⇒ 0
          (--every (= it it-index) '(0 1 3))
              ⇒ nil

 -- 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? #'numberp '(nil 0 t))
              ⇒ t
          (-any? #'numberp '(nil t t))
              ⇒ 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’.
     In the latter case, stop after the first X for which (PRED X) is
     ‘nil’, without calling PRED on any subsequent elements of LIST.

     The similar function ‘-every’ (*note -every::) is more widely
     useful, since it returns the last non-‘nil’ result of PRED instead
     of just ‘t’ on success.

     Alias: ‘-all-p’, ‘-every-p’, ‘-every?’.

     This function’s anaphoric counterpart is ‘--all?’.

          (-all? #'numberp '(1 2 3))
              ⇒ t
          (-all? #'numberp '(2 t 6))
              ⇒ nil
          (--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 different LIST items both satisfy and do not satisfy
     PRED.  That is, if PRED returns both ‘nil’ for at least one item,
     and non-‘nil’ for at least one other item in LIST.  Return ‘nil’ if
     all items satisfy the predicate or none of them do.

     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 a 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 a 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)
     Split LIST into a prefix satisfying PRED, and the rest.  The first
     sublist is the prefix of LIST with successive elements satisfying
     PRED, and the second sublist is the remaining elements that do not.
     The result is like performing

     ((-take-while PRED LIST) (-drop-while PRED LIST))

     but in no more than a single pass through 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)
     Split LIST into two sublists based on whether items satisfy PRED.
     The result is like performing

     ((-filter PRED LIST) (-remove PRED LIST))

     but in a single pass through 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)
     Partition LIST into sublists of length N that are STEP items apart.
     Like ‘-partition-all-in-steps’ (*note -partition-all-in-steps::),
     but 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)
     Partition LIST into sublists of length N that are STEP items apart.
     Adjacent groups may overlap if N exceeds the STEP stride.  Trailing
     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 LIST after each element for which PRED returns non-‘nil’.

     This function’s anaphoric counterpart is ‘--partition-after-pred’.

          (-partition-after-pred #'booleanp ())
              ⇒ ()
          (-partition-after-pred #'booleanp '(t t))
              ⇒ ((t) (t))
          (-partition-after-pred #'booleanp '(0 0 t t 0 t))
              ⇒ ((0 0 t) (t) (0 t))

 -- Function: -partition-before-pred (pred list)
     Partition directly before each time PRED is true on an element of
     LIST.

          (-partition-before-pred #'booleanp ())
              ⇒ ()
          (-partition-before-pred #'booleanp '(0 t))
              ⇒ ((0) (t))
          (-partition-before-pred #'booleanp '(0 0 t 0 t t))
              ⇒ ((0 0) (t 0) (t) (t))

 -- 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
============

Functions retrieving or sorting based on list indices and related
predicates.

 -- 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 of all elements appearing in either LIST1 or
     LIST2.  Equality is defined by the value of ‘-compare-fn’ if
     non-‘nil’; otherwise ‘equal’.

          (-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 of the elements appearing in both LIST1 and
     LIST2.  Equality is defined by the value of ‘-compare-fn’ if
     non-‘nil’; otherwise ‘equal’.

          (-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 (left if N is negative).  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 any ELEMENTS are given, append them 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: -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::).

          (-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 (list)
     Return the second item of LIST, or ‘nil’ if LIST is too short.

     See also: ‘-third-item’ (*note -third-item::).

          (-second-item '(1 2 3))
              ⇒ 2
          (-second-item nil)
              ⇒ nil

 -- Function: -third-item (list)
     Return the third item of LIST, or ‘nil’ if LIST is too short.

     See also: ‘-fourth-item’ (*note -fourth-item::).

          (-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 (arg)
     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 ())
              ⇒ ()
          (-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) "}" '((elisp-mode (foo (bar . booze)) (baz . qux)) (c-mode (foo . bla) (bum . bam)))))
              ⇒ "{elisp-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
=====================

Macros that conditionally combine sequential forms for brevity or
readability.

 -- 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--> (expr &rest forms)
     Thread EXPR through FORMS via ‘-->’ (*note -->::), while the result
     is non-‘nil’.  When EXPR evaluates to non-‘nil’, thread the result
     through the first of FORMS, and when that result is non-‘nil’,
     thread it through the next form, etc.

          (-some--> "def" (concat "abc" it "ghi"))
              ⇒ "abcdefghi"
          (-some--> nil (concat "abc" it "ghi"))
              ⇒ nil
          (-some--> '(0 1) (-remove #'natnump it) (append it it) (-map #'1+ it))
              ⇒ ()

 -- 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
============

Macros that combine ‘let’ and ‘let*’ with destructuring and 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::).

          (-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::).

          (-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 ([match-form val] ...)
     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.

          (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) (< it 5) (setq s (+ s it))) s)
              ⇒ 8

 -- 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 () (push it l)) l)
              ⇒ ()

 -- 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) (> it 1) (setq s (+ s it))) s)
              ⇒ 10

 -- 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)
              ⇒ ()
          (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
===========================

Macros that modify variables holding lists.

 -- 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
=========================

Functions that manipulate and compose other functions.

 -- Function: -partial (fun &rest args)
     Return a function that is a partial application of FUN to ARGS.
     ARGS is a list of the first N arguments to pass to FUN.  The result
     is a new function which does the same as FUN, except that the first
     N arguments are fixed at the values with which this function was
     called.

          (funcall (-partial #'+ 5))
              ⇒ 5
          (funcall (-partial #'- 5) 3)
              ⇒ 2
          (funcall (-partial #'+ 5 2) 3)
              ⇒ 10

 -- Function: -rpartial (fn &rest args)
     Return a function that is a partial application of FN to ARGS.
     ARGS is a list of the last N arguments to pass to FN.  The result
     is a new function which does the same as FN, except that the last N
     arguments are fixed at the values with which this function was
     called.  This is like ‘-partial’ (*note -partial::), except the
     arguments are fixed starting from the right rather than the left.

          (funcall (-rpartial #'- 5))
              ⇒ -5
          (funcall (-rpartial #'- 5) 8)
              ⇒ 3
          (funcall (-rpartial #'- 5 2) 10)
              ⇒ 3

 -- Function: -juxt (&rest fns)
     Return a function that is the juxtaposition of FNS.  The returned
     function takes a variable number of ARGS, applies each of FNS in
     turn to ARGS, and returns the list of results.

          (funcall (-juxt) 1 2)
              ⇒ ()
          (funcall (-juxt #'+ #'- #'* #'/) 7 5)
              ⇒ (12 2 35 1)
          (mapcar (-juxt #'number-to-string #'1+) '(1 2))
              ⇒ (("1" 2) ("2" 3))

 -- Function: -compose (&rest fns)
     Compose FNS into a single composite function.  Return a function
     that takes a variable number of ARGS, applies the last function in
     FNS to ARGS, and returns the result of calling each remaining
     function on the result of the previous function, right-to-left.  If
     no FNS are given, return a variadic ‘identity’ function.

          (funcall (-compose #'- #'1+ #'+) 1 2 3)
              ⇒ -7
          (funcall (-compose #'identity #'1+) 3)
              ⇒ 4
          (mapcar (-compose #'not #'stringp) '(nil ""))
              ⇒ (t nil)

 -- Function: -applify (fn)
     Return a function that applies FN to a single list of args.  This
     changes the arity of FN from taking N distinct arguments to taking
     1 argument which is a list of N arguments.

          (funcall (-applify #'+) nil)
              ⇒ 0
          (mapcar (-applify #'+) '((1 1 1) (1 2 3) (5 5 5)))
              ⇒ (3 6 15)
          (funcall (-applify #'<) '(3 6))
              ⇒ t

 -- Function: -on (op trans)
     Return a function that calls TRANS on each arg and OP on the
     results.  The returned function takes a variable number of
     arguments, calls the function TRANS on each one in turn, and then
     passes those results as the list of arguments to OP, in the same
     order.

     For example, the following pairs of expressions are morally
     equivalent:

     (funcall (-on #’+ #’1+) 1 2 3) = (+ (1+ 1) (1+ 2) (1+ 3)) (funcall
     (-on #’+ #’1+)) = (+)

          (-sort (-on #'< #'length) '((1 2 3) (1) (1 2)))
              ⇒ ((1) (1 2) (1 2 3))
          (funcall (-on #'min #'string-to-number) "22" "2" "1" "12")
              ⇒ 1
          (-min-by (-on #'> #'length) '((1 2 3) (4) (1 2)))
              ⇒ (4)

 -- Function: -flip (fn)
     Return a function that calls FN with its arguments reversed.  The
     returned function takes the same number of arguments as FN.

     For example, the following two expressions are morally equivalent:

     (funcall (-flip #’-) 1 2) = (- 2 1)

     See also: ‘-rotate-args’ (*note -rotate-args::).

          (-sort (-flip #'<) '(4 3 6 1))
              ⇒ (6 4 3 1)
          (funcall (-flip #'-) 3 2 1 10)
              ⇒ 4
          (funcall (-flip #'1+) 1)
              ⇒ 2

 -- Function: -rotate-args (n fn)
     Return a function that calls FN with args rotated N places to the
     right.  The returned function takes the same number of arguments as
     FN, rotates the list of arguments N places to the right (left if N
     is negative) just like ‘-rotate’ (*note -rotate::), and applies FN
     to the result.

     See also: ‘-flip’ (*note -flip::).

          (funcall (-rotate-args -1 #'list) 1 2 3 4)
              ⇒ (2 3 4 1)
          (funcall (-rotate-args 1 #'-) 1 10 100)
              ⇒ 89
          (funcall (-rotate-args 2 #'list) 3 4 5 1 2)
              ⇒ (1 2 3 4 5)

 -- 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
          (mapcar (-const 1) '("a" "b" "c" "d"))
              ⇒ (1 1 1 1)
          (-sum (mapcar (-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] "\1\2\3")

 -- Function: -not (pred)