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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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31
src/libexpr/primops.cc
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@ -2149,21 +2149,27 @@ static void prim_listToAttrs(EvalState & state, const Pos & pos, Value * * args,
state.forceAttrs(v2, pos);
Bindings::iterator j = v2.attrs->find(state.sName);
if (j == v2.attrs->end())
throw TypeError({
.msg = hintfmt("'name' attribute missing in a call to 'listToAttrs'"),
.errPos = pos
if (j == v2.attrs->end()) {
auto e = TypeError({
.msg = hintfmt("'name' attribute missing for 'listToAttrs'"),
.errPos = *v2.attrs->pos
});
string name = state.forceStringNoCtx(*j->value, pos);
e.addTrace(pos, hintfmt("while invoking '%s'", "listToAttrs"));
throw e;
}
string name = state.forceStringNoCtx(*j->value, *j->pos);
Symbol sym = state.symbols.create(name);
if (seen.insert(sym).second) {
Bindings::iterator j2 = v2.attrs->find(state.symbols.create(state.sValue));
if (j2 == v2.attrs->end())
throw TypeError({
.msg = hintfmt("'value' attribute missing in a call to 'listToAttrs'"),
.errPos = pos
if (j2 == v2.attrs->end()) {
auto e = TypeError({
.msg = hintfmt("'value' attribute missing for 'listToAttrs'"),
.errPos = *v2.attrs->pos
});
e.addTrace(pos, hintfmt("while invoking '%s'", "listToAttrs"));
throw e;
}
v.attrs->push_back(Attr(sym, j2->value, j2->pos));
}
}
@ -2804,7 +2810,12 @@ static void prim_concatMap(EvalState & state, const Pos & pos, Value * * args, V
for (unsigned int n = 0; n < nrLists; ++n) {
Value * vElem = args[1]->listElems()[n];
state.callFunction(*args[0], *vElem, lists[n], pos);
state.forceList(lists[n], pos);
try {
state.forceList(lists[n], lists[n].determinePos(args[0]->determinePos(pos)));
} catch (TypeError &e) {
e.addTrace(pos, hintfmt("while invoking '%s'", "concatMap"));
throw e;
}
len += lists[n].listSize();
}