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68 lines
3.2 KiB
Nix
68 lines
3.2 KiB
Nix
# This file defines a single function for booting a package set from a list of
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# stages. The exact mechanics of that function are defined below; here I
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# (@Ericson2314) wish to describe the purpose of the abstraction.
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#
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# The first goal is consistency across stdenvs. Regardless of what this function
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# does, by making every stdenv use it for bootstrapping we ensure that they all
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# work in a similar way. [Before this abstraction, each stdenv was its own
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# special snowflake due to different authors writing in different times.]
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#
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# The second goal is consistency across each stdenv's stage functions. By
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# writing each stage it terms of the previous stage, commonalities between them
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# are more easily observable. [Before, there usually was a big attribute set
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# with each stage, and stages would access the previous stage by name.]
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#
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# The third goal is composition. Because each stage is written in terms of the
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# previous, the list can be reordered or, more practically, extended with new
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# stages. The latter is used for cross compiling and custom
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# stdenvs. Additionally, certain options should by default apply only to the
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# last stage, whatever it may be. By delaying the creation of stage package sets
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# until the final fold, we prevent these options from inhibiting composition.
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#
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# The fourth and final goal is debugging. Normal packages should only source
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# their dependencies from the current stage. But for the sake of debugging, it
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# is nice that all packages still remain accessible. We make sure previous
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# stages are kept around with a `stdenv.__bootPackges` attribute referring the
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# previous stage. It is idiomatic that attributes prefixed with `__` come with
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# special restrictions and should not be used under normal circumstances.
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{ lib, allPackages }:
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# Type:
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# [ pkgset -> (args to stage/default.nix) or ({ __raw = true; } // pkgs) ]
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# -> pkgset
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#
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# In english: This takes a list of function from the previous stage pkgset and
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# returns the final pkgset. Each of those functions returns, if `__raw` is
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# undefined or false, args for this stage's pkgset (the most complex and
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# important arg is the stdenv), or, if `__raw = true`, simply this stage's
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# pkgset itself.
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#
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# The list takes stages in order, so the final stage is last in the list. In
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# other words, this does a foldr not foldl.
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stageFuns: let
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# Take the list and disallow custom overrides in all but the final stage,
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# and allow it in the final flag. Only defaults this boolean field if it
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# isn't already set.
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withAllowCustomOverrides = lib.lists.imap
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(index: stageFun: prevStage:
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# So true by default for only the first element because one
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# 1-indexing. Since we reverse the list, this means this is true
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# for the final stage.
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{ allowCustomOverrides = index == 1; }
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// (stageFun prevStage))
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(lib.lists.reverseList stageFuns);
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# Adds the stdenv to the arguments, and sticks in it the previous stage for
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# debugging purposes.
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folder = stageFun: finalSoFar: let
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args = stageFun finalSoFar;
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stdenv = args.stdenv // {
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# For debugging
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__bootPackages = finalSoFar;
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};
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args' = args // { inherit stdenv; };
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in
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(if args.__raw or false then lib.id else allPackages) args';
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in lib.lists.fold folder {} withAllowCustomOverrides
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