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# This file defines a single function for booting a package set from a list of
# stages. The exact mechanics of that function are defined below; here I
# (@Ericson2314) wish to describe the purpose of the abstraction.
#
# The first goal is consistency across stdenvs. Regardless of what this function
# does, by making every stdenv use it for bootstrapping we ensure that they all
# work in a similar way. [Before this abstraction, each stdenv was its own
# special snowflake due to different authors writing in different times.]
#
# The second goal is consistency across each stdenv's stage functions. By
# writing each stage it terms of the previous stage, commonalities between them
# are more easily observable. [Before, there usually was a big attribute set
# with each stage, and stages would access the previous stage by name.]
#
# The third goal is composition. Because each stage is written in terms of the
# previous, the list can be reordered or, more practically, extended with new
# stages. The latter is used for cross compiling and custom
# stdenvs. Additionally, certain options should by default apply only to the
# last stage, whatever it may be. By delaying the creation of stage package sets
# until the final fold, we prevent these options from inhibiting composition.
#
# The fourth and final goal is debugging. Normal packages should only source
# their dependencies from the current stage. But for the sake of debugging, it
# is nice that all packages still remain accessible. We make sure previous
# stages are kept around with a `stdenv.__bootPackges` attribute referring the
# previous stage. It is idiomatic that attributes prefixed with `__` come with
# special restrictions and should not be used under normal circumstances.
{ lib, allPackages }:
# Type:
# [ pkgset -> (args to stage/default.nix) or ({ __raw = true; } // pkgs) ]
# -> pkgset
#
# In english: This takes a list of function from the previous stage pkgset and
# returns the final pkgset. Each of those functions returns, if `__raw` is
# undefined or false, args for this stage's pkgset (the most complex and
# important arg is the stdenv), or, if `__raw = true`, simply this stage's
# pkgset itself.
#
# The list takes stages in order, so the final stage is last in the list. In
# other words, this does a foldr not foldl.
stageFuns: let
/* "dfold" a ternary function `op' between successive elements of `list' as if
it was a doubly-linked list with `lnul' and `rnul` base cases at either
end. In precise terms, `fold op lnul rnul [x_0 x_1 x_2 ... x_n-1]` is the
same as
let
f_-1 = lnul;
f_0 = op f_-1 x_0 f_1;
f_1 = op f_0 x_1 f_2;
f_2 = op f_1 x_2 f_3;
...
f_n = op f_n-1 x_n f_n+1;
f_n+1 = rnul;
in
f_0
*/
dfold = op: lnul: rnul: list:
let
len = builtins.length list;
go = pred: n:
if n == len
then rnul
else let
# Note the cycle -- call-by-need ensures finite fold.
cur = op pred (builtins.elemAt list n) succ;
succ = go cur (n + 1);
in cur;
in go lnul 0;
# Take the list and disallow custom overrides in all but the final stage,
# and allow it in the final flag. Only defaults this boolean field if it
# isn't already set.
withAllowCustomOverrides = lib.lists.imap1
(index: stageFun: prevStage:
# So true by default for only the first element because one
# 1-indexing. Since we reverse the list, this means this is true
# for the final stage.
{ allowCustomOverrides = index == 1; }
// (stageFun prevStage))
(lib.lists.reverseList stageFuns);
# Adds the stdenv to the arguments, and sticks in it the previous stage for
# debugging purposes.
folder = nextStage: stageFun: prevStage: let
args = stageFun prevStage;
args' = args // {
stdenv = args.stdenv // {
# For debugging
__bootPackages = prevStage;
__hatPackages = nextStage;
};
};
in
if args.__raw or false
then args'
else allPackages ((builtins.removeAttrs args' ["selfBuild"]) // {
buildPackages = if args.selfBuild or true then null else prevStage;
targetPackages = if args.selfBuild or true then null else nextStage;
});
in dfold folder {} {} withAllowCustomOverrides
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