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How are set argument to nix-expression

I'm new to Nix and I'm trying to understand the hello derivation given in example.

I can understand the syntax and what is supposed to do, however I don't understand how the initial arguments (and the especially the perl one_ are fed ?

I mean, who is setting the perl argument before calling this derivation. Does that mean that perl is a dependency of hello ?

Solution

  • Packages are typically written as set of dependencies -> derivation functions, to be assembled later. The arguments you ask about are fed from pkgs/top-level/all-packages.nix, which holds the set of all packages in Nixpkgs.

    When you find the hello's line in all-packages.nix, you'll notice it's using callPackage - it's signature is path to Nix expression -> overrides -> derivation. callPackage loads the path, looks at the function it loaded, and for each arguments provides either value from overrides or, if not given, from the huge set in all-packages.nix.

    For a nice description of callPackage see http://lethalman.blogspot.com/2014/09/nix-pill-13-callpackage-design-pattern.html - it's a less condensed explanation, showing how you could have invented callPackage yourself :-).