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libexpr: misc improvements for proper error position

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When working on some more complex Nix code, there are sometimes rather
unhelpful or misleading error messages, especially if coerce-errors are
thrown.

This patch is a first steps towards improving that. I'm happy to file
more changes after that, but I'd like to gather some feedback first.

To summarize, this patch does the following things:

* Attrsets (a.k.a. `Bindings` in `libexpr`) now have a `Pos`. This is
  helpful e.g. to identify which attribute-set in `listToAttrs` is
  invalid.

* The `Value`-struct has a new method named `determinePos` which tries
  to guess the position of a value and falls back to a default if that's
  not possible.

  This can be used to provide better messages if a coercion fails.

* The new `determinePos`-API is used by `builtins.concatMap` now. With
  that change, Nix shows the exact position in the error where a wrong
  value was returned by the lambda.

  To make sure it's still obvious that `concatMap` is the problem,
  another stack-frame was added.

* The changes described above can be added to every other `primop`, but
  first I'd like to get some feedback about the overall approach.
This commit is contained in:
Maximilian Bosch 2021-01-08 22:27:00 +01:00
parent 3550a32b25
commit 7c76964daa
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GPG key ID: 091DBF4D1FC46B8E
6 changed files with 39 additions and 12 deletions
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11
src/libexpr/eval.cc
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@ -201,6 +201,15 @@ string showType(const Value & v)
}
}
Pos Value::determinePos(const Pos &pos) const
{
switch (internalType) {
case tAttrs: return *attrs->pos;
case tLambda: return lambda.fun->pos;
case tApp: return app.left->determinePos(pos);
default: return pos;
}
}
bool Value::isTrivial() const
{
@ -1060,6 +1069,8 @@ void ExprAttrs::eval(EvalState & state, Env & env, Value & v)
v.attrs->push_back(Attr(nameSym, i.valueExpr->maybeThunk(state, *dynamicEnv), &i.pos));
v.attrs->sort(); // FIXME: inefficient
}
v.attrs->pos = &pos;
}