How do I patch agda for use in a nix shell?

Help
3

Thank you, replacing agda with haskellPackages.agda did work.

with import <nixpkgs> { };

let
  my-agda = agdaPackages.override {
    Agda = haskellPackages.Agda.overrideAttrs {
      patches = [ (fetchpatch { ... }) ];
    };
  }.agda.withPackages (p: [ p.standard-library ]);
in

mkShell {
  buildInputs = [ my-agda ];
}

Strange though. Could a nix package rebuild even if its source was unchanged? I remember well that I never attempted this solution before, and I remember just as well agda building from source.