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auto-call error

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This commit is contained in:
Ben Burdette 2020-11-09 16:48:35 -07:00
parent dc7d1322ef
commit 107c91f5fe
1 changed files with 22 additions and 1 deletions
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23
src/libexpr/eval.cc
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@ -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);
}
}
}