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nix flake check: improve error message if overlay is not a lambda (#8582)

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* nix flake check: improve error message if overlay is not a lambda

Suppose you have an overlay like this

    {
      inputs = { /* ... */ };
      outputs = { flake-utils, ... }: flake-utils.lib.eachDefaultSystem
        (system: {
          overlays.default = final: prev: {

          };
        });
    }

then `nix flake check` (correctly) fails because `overlays` are supposed
to have the structure `overlays.<name> = final: prev: exp`. However, the
error-message is a little bit counter-intuitive:

    error: overlay does not take an argument named 'final'

While one might guess where the error actually comes from because the
trace above says `… while checking the overlay 'overlays.x86_64-linux'`
this is still pretty confusing because it complains about an argument
not being named `final` even though that's evidently the case.

With this change, the error-message actually makes it clear what's
wrong:

    [ma27@carsten:~/Projects/nix/tmp]$ nix flake check --extra-experimental-features 'nix-command flakes' path:$(pwd)
    error:
           … while checking flake output 'overlays'

             at /nix/store/clgblnxx003hyrq8qkz5ab6kgqkck6qc-source/flake.nix:4:5:

                3|   outputs = { ... }: {
                4|     overlays.x86_64-linux.snens = final: prev: {
                 |     ^
                5|       kek = throw "snens";

           … while checking the overlay 'overlays.x86_64-linux'

             at /nix/store/clgblnxx003hyrq8qkz5ab6kgqkck6qc-source/flake.nix:4:5:

                3|   outputs = { ... }: {
                4|     overlays.x86_64-linux.snens = final: prev: {
                 |     ^
                5|       kek = throw "snens";

           error: overlay is not a lambda, but a set instead
This commit is contained in:
Maximilian Bosch 2023-06-27 14:58:29 +02:00 • committed by GitHub
parent 8e770dac9f
commit 559fd7ffe7
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2 changed files with 16 additions and 2 deletions
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6
src/nix/flake.cc
View file

@ -386,8 +386,10 @@ struct CmdFlakeCheck : FlakeCommand
auto checkOverlay = [&](const std::string & attrPath, Value & v, const PosIdx pos) {
try {
state->forceValue(v, pos);
if (!v.isLambda()
|| v.lambda.fun->hasFormals()
if (!v.isLambda()) {
throw Error("overlay is not a function, but %s instead", showType(v));
}
if (v.lambda.fun->hasFormals()
|| !argHasName(v.lambda.fun->arg, "final"))
throw Error("overlay does not take an argument named 'final'");
auto body = dynamic_cast<ExprLambda *>(v.lambda.fun->body);

12
tests/flakes/check.sh
View file

@ -25,6 +25,18 @@ EOF
(! nix flake check $flakeDir)
cat > $flakeDir/flake.nix <<EOF
{
outputs = { self, ... }: {
overlays.x86_64-linux.foo = final: prev: {
};
};
}
EOF
checkRes=$(nix flake check $flakeDir 2>&1 && fail "nix flake check --all-systems should have failed" || true)
echo "$checkRes" | grepQuiet "error: overlay is not a function, but a set instead"
cat > $flakeDir/flake.nix <<EOF
{
outputs = { self }: {