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notebook/notes/nix/callPackage.md

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title TARGET DECK FILE TAGS tags
callPackage Obsidian::STEM nix
nix

Overview

We first examine lib.makeOverridable. It's implementation isn't too important but understanding how it's used is. We adapt the example found in nixpkgs:

nix-repl> x = {a, b}: { example = a + b; }
nix-repl> y = lib.makeOverridable x { a = 1; b = 2; }
nix-repl> y
{ override = «lambda»; overrideDerivation = «lambda»; example = 3; }
nix-repl> y.override { a = 10; }
{ override = «lambda»; overrideDerivation = «lambda»; example = 12; }

lib.makeOverridable is an example of partial function application. Notice y still contains the result (example) of evaluating x. We can re-run the computation with a different value (in this case a) by invoking y.override.

%%ANKI Basic What is the result of the following?

x = {a, b}: { example = a + b; }
lib.functionArgs x

Back: { a = false; b = false; } Reference: Yin, Ryan. “NixOS and Flakes Book.” Nix, February 1, 2024. https://github.com/ryan4yin/nixos-and-flakes-book

END%%

%%ANKI Basic What does each boolean returned by lib.functionArgs indicate? Back: Whether the associated attribute has a default value. Reference: Yin, Ryan. “NixOS and Flakes Book.” Nix, February 1, 2024. https://github.com/ryan4yin/nixos-and-flakes-book

END%%

%%ANKI Basic What additional attributes is included in the set returned by lib.makeOverridable? Back: override and overrideDerivation. Reference: Yin, Ryan. “NixOS and Flakes Book.” Nix, February 1, 2024. https://github.com/ryan4yin/nixos-and-flakes-book

END%%

%%ANKI Basic What is the value of y.example in the following?

x = {a, b}: { example = a + b; }
y = lib.makeOverridable x { a = 1; b = 2; }

Back: 3 Reference: Yin, Ryan. “NixOS and Flakes Book.” Nix, February 1, 2024. https://github.com/ryan4yin/nixos-and-flakes-book

END%%

%%ANKI Basic What is the value of (y.override { a = 10; }).example in the following?

x = {a, b}: { example = a + b; }
y = lib.makeOverridable x { a = 1; b = 2; }

Back: 12 Reference: Yin, Ryan. “NixOS and Flakes Book.” Nix, February 1, 2024. https://github.com/ryan4yin/nixos-and-flakes-book

END%%

Now we can understand how pkgs.callPackage works. The following is a simplification of the actual implementation:

callPackageWith = autoArgs: fn: args:
  let
    f = if isFunction fn then fn else import fn;
    fargs = functionArgs f;
    allArgs = intersectArgs fargs autoArgs // args;
  in
    lib.makeOverridable f allArgs

callPackage = callPackageWith pkgs;

%%ANKI Basic What two functions is callPackage implemented on top of? Back: callPackageWith and makeOverridable. Reference: Yin, Ryan. “NixOS and Flakes Book.” Nix, February 1, 2024. https://github.com/ryan4yin/nixos-and-flakes-book

END%%

%%ANKI Basic What is the purpose of callPackage? Back: It calls package functions with arguments automatically supplied if not overridden. Reference: Yin, Ryan. “NixOS and Flakes Book.” Nix, February 1, 2024. https://github.com/ryan4yin/nixos-and-flakes-book

END%%

%%ANKI Basic What attribute must be invoked in callPackage's return value to override arguments? Back: override Reference: Yin, Ryan. “NixOS and Flakes Book.” Nix, February 1, 2024. https://github.com/ryan4yin/nixos-and-flakes-book

END%%

References