Where is `callPackage` defined in the Nixpkgs repo (or how to find Nix lambda definitions in general)?

Question

Found a lot of sources describing callPackage and some of its internals, but none that refers to it's location. It's like the best kept secret of Nix, and the manuals even seem to be actively avoiding the topic. I could find it given time, but it's a huge repo.

Resources:

• How does Nix's “callPackage” call functions defined without an ellipsis? (Stackoverlow)
• Nix Pills, Chapter 13. Callpackage Design Pattern Nix

---

The answer actually points to callPackageWith function in customisation.nix, but callPackage itself is defined elsewhere.

Answer 1

nix repl can tell you the location where a lambda is defined.

$ nix repl
Welcome to Nix version 2.2.2. Type :? for help.

nix-repl> pkgs = import <nixpkgs> {}

nix-repl> pkgs.callPackage
«lambda @ /nix/store/9hffpjaa2a7djl19ncky7zcvlhyj76dn-nixos-19.03.172396.d740b2ee855/nixos/lib/customisation.nix:108:31»

where the part up to and including nixos is my <nixpkgs>, thus giving you the answer: lib/customisation.nix line 108.