diff options
Diffstat (limited to 'src/libexpr/eval.cc')
| -rw-r--r-- | src/libexpr/eval.cc | 23 |
1 files changed, 22 insertions, 1 deletions
diff --git a/src/libexpr/eval.cc b/src/libexpr/eval.cc index d6366050c..e52e8dcf2 100644 --- a/src/libexpr/eval.cc +++ b/src/libexpr/eval.cc @@ -1370,7 +1370,28 @@ void EvalState::autoCallFunction(Bindings & args, Value & fun, Value & res) if (j != args.end()) { actualArgs->attrs->push_back(*j); } else if (!i.def) { - throwTypeError("cannot auto-call a function that has an argument without a default value ('%1%')", i.name); + throwUndefinedVarError(R"(cannot auto-call a function that has an argument without a default value ('%1%') + An 'auto-call' is when a nix expression is evaluated without any external arguments. If that + nix expression is a function, and that function's arguments all have default values, then all is well. + + But if the function arguments don't have default values, then evaluation fails. + + The classic case for this error is evaluating a nix file with nix-build that expects to be evaluated by callPackage. + # in 'callPackage' format: expression is a function that takes an argument 'stdenv'. + # callPackage would implicitly pull 'stdenv' from nixpkgs, then call this function. + { stdenv }: + stdenv.mkDerivation { + ... + + # in 'auto-call' format: nixpkgs is imported explicitly, and used directly. + let + nixpkgs = import <nixpkgs> {}; + in + nixpkgs.stdenv.mkDerivation { + ... + + See this nix pill for more information re callPackage format: + https://nixos.org/guides/nix-pills/callpackage-design-pattern.html)", i.name); } } } |