Shea told me composableDerivation is hard to understand.

Thus comment it and why it was written long time ago until it'll get replaced piecewise.
nixos-19.03
Marc Weber 9 years ago
parent bfc524664a
commit 31fc4c6aa4
  1. 77
      lib/composable-derivation.nix

@ -1,15 +1,74 @@
{lib, pkgs} :
let inherit (lib) nv nvs; in
{
# see for example:
# - development/interpreters/php_configurable/default.nix
# - .. search composableDerivation in all-packages.nix ..
#
# You should be able to override anything you like easily
# grep the mailinglist by title "python proposal" (dec 08)
# -> http://mail.cs.uu.nl/pipermail/nix-dev/2008-December/001571.html
# to see why this got complicated when using all its features
# TODO add newer example using new syntax (kernel derivation proposal -> mailinglist)
# composableDerivation basically mixes these features:
# - fix function
# - mergeAttrBy
# - provides shortcuts for "options" such as "--enable-foo" and adding
# buildInputs, see php example
#
# It predates styles which are common today, such as
# * the config attr
# * mkDerivation.override feature
# * overrideDerivation (lib/customization.nix)
#
# Some of the most more important usage examples (which could be rewritten if it was important):
# * php
# * postgis
# * vim_configurable
#
# A minimal example illustrating most features would look like this:
# let base = composableDerivation { (fixed : let inherit (fixed.fixed) name in {
# src = fetchurl {
# }
# buildInputs = [A];
# preConfigre = "echo ${name}";
# # attention, "name" attr is missing, thus you cannot instantiate "base".
# }
# in {
# # These all add name attribute, thus you can instantiate those:
# v1 = base.merge ({ name = "foo-add-B"; buildInputs = [B]; }); // B gets merged into buildInputs
# v2 = base.merge ({ name = "mix-in-pre-configure-lines" preConfigre = ""; });
# v3 = base.replace ({ name = "foo-no-A-only-B;" buildInputs = [B]; });
# }
#
# So yes, you can think about it being something like nixos modules, and
# you'd be merging "features" in one at a time using .merge or .replace
# Thanks Shea for telling me that I rethink the documentation ..
#
# issues:
# * its complicated to understand
# * some "features" such as exact merge behaviour are burried in mergeAttrBy
# and defaultOverridableDelayableArgs assuming the default behaviour does
# the right thing in the common case
# * Eelco once said using such fix style functions are slow to evaluate
# * Too quick & dirty. Hard to understand for others. The benefit was that
# you were able to create a kernel builder like base derivation and replace
# / add patches the way you want without having to declare function arguments
#
# nice features:
# declaring "optional featuers" is modular. For instance:
# flags.curl = {
# configureFlags = ["--with-curl=${curl}" "--with-curlwrappers"];
# buildInputs = [curl openssl];
# };
# flags.other = { .. }
# (Example taken from PHP)
#
# alternative styles / related features:
# * Eg see function supporting building the kernel
# * versionedDerivation (discussion about this is still going on - or ended)
# * composedArgsAndFun
# * mkDerivation.override
# * overrideDerivation
# * using { .., *Support ? false }: like configurable options.
# To find those examples use grep
#
# To sum up: It exists for historical reasons - and for most commonly used
# tasks the alternatives should be used
#
# If you have questions about this code ping Marc Weber.
composableDerivation = {
mkDerivation ? pkgs.stdenv.mkDerivation,

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