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libexpr: improve error messages for builtins.genericClosure

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Show which element(s) are involved at each error point:

- When an element is missing the "key" attribute, show the element
- When an element is not an attribute set, show the element
- When comparing keys fails, show both elements being compared
- When calling operator fails, show which element was being processed

This provides concrete context using ValuePrinter with errorPrintOptions.

Note: errorPrintOptions uses maxDepth=10 by default, which may print
quite deeply nested structures in error messages. This could potentially
be overwhelming, but follows the existing default for error contexts.
This commit is contained in:
Robert Hensing 2025-11-06 21:30:40 +01:00
parent ca787bc3e0
commit d262efc240
10 changed files with 188 additions and 46 deletions
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100
src/libexpr/primops.cc
View file

@ -681,7 +681,14 @@ struct CompareValues
if (v1->type() == nInt && v2->type() == nFloat)
return v1->integer().value < v2->fpoint();
if (v1->type() != v2->type())
state.error<EvalError>("cannot compare %s with %s", showType(*v1), showType(*v2)).debugThrow();
state
.error<EvalError>(
"cannot compare %s with %s; values are %s and %s",
showType(*v1),
showType(*v2),
ValuePrinter(state, *v1, errorPrintOptions),
ValuePrinter(state, *v2, errorPrintOptions))
.debugThrow();
// Allow selecting a subset of enum values
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wswitch-enum"
@ -711,7 +718,11 @@ struct CompareValues
default:
state
.error<EvalError>(
"cannot compare %s with %s; values of that type are incomparable", showType(*v1), showType(*v2))
"cannot compare %s with %s; values of that type are incomparable (values are %s and %s)",
showType(*v1),
showType(*v2),
ValuePrinter(state, *v1, errorPrintOptions),
ValuePrinter(state, *v2, errorPrintOptions))
.debugThrow();
#pragma GCC diagnostic pop
}
@ -757,42 +768,79 @@ static void prim_genericClosure(EvalState & state, const PosIdx pos, Value ** ar
`workSet', adding the result to `workSet', continuing until
no new elements are found. */
ValueList res;
// `doneKeys' doesn't need to be a GC root, because its values are
// reachable from res.
auto cmp = CompareValues(state, noPos, "while comparing the `key` attributes of two genericClosure elements");
std::set<Value *, decltype(cmp)> doneKeys(cmp);
// Track which element each key came from
auto cmp = CompareValues(state, noPos, "");
std::map<Value *, Value *, decltype(cmp)> keyToElem(cmp);
while (!workSet.empty()) {
Value * e = *(workSet.begin());
workSet.pop_front();
state.forceAttrs(
*e,
noPos,
"while evaluating one of the elements generated by (or initially passed to) builtins.genericClosure");
try {
state.forceAttrs(*e, noPos, "");
} catch (Error & err) {
err.addTrace(nullptr, "in genericClosure element %s", ValuePrinter(state, *e, errorPrintOptions));
throw;
}
auto key = state.getAttr(
state.s.key,
e->attrs(),
"in one of the attrsets generated by (or initially passed to) builtins.genericClosure");
const Attr * key;
try {
key = state.getAttr(state.s.key, e->attrs(), "");
} catch (Error & err) {
err.addTrace(nullptr, "in genericClosure element %s", ValuePrinter(state, *e, errorPrintOptions));
throw;
}
state.forceValue(*key->value, noPos);
if (!doneKeys.insert(key->value).second)
continue;
try {
auto [it, inserted] = keyToElem.insert({key->value, e});
if (!inserted)
continue;
} catch (Error & err) {
// Try to find which element we're comparing against
Value * otherElem = nullptr;
for (auto & [otherKey, elem] : keyToElem) {
try {
cmp(key->value, otherKey);
} catch (Error &) {
// Found the element we're comparing against
otherElem = elem;
break;
}
}
if (otherElem) {
// Traces are printed in reverse order; pre-swap them.
err.addTrace(nullptr, "with element %s", ValuePrinter(state, *otherElem, errorPrintOptions));
err.addTrace(nullptr, "while comparing element %s", ValuePrinter(state, *e, errorPrintOptions));
} else {
// Couldn't find the specific element, just show current
err.addTrace(nullptr, "while checking key of element %s", ValuePrinter(state, *e, errorPrintOptions));
}
throw;
}
res.push_back(e);
/* Call the `operator' function with `e' as argument. */
Value newElements;
state.callFunction(*op->value, {&e, 1}, newElements, noPos);
state.forceList(
newElements,
noPos,
"while evaluating the return value of the `operator` passed to builtins.genericClosure");
try {
state.callFunction(*op->value, {&e, 1}, newElements, noPos);
state.forceList(
newElements,
noPos,
"while evaluating the return value of the `operator` passed to builtins.genericClosure");
/* Add the values returned by the operator to the work set. */
for (auto elem : newElements.listView()) {
state.forceValue(*elem, noPos); // "while evaluating one one of the elements returned by the `operator`
// passed to builtins.genericClosure");
workSet.push_back(elem);
/* Add the values returned by the operator to the work set. */
for (auto elem : newElements.listView()) {
state.forceValue(*elem, noPos); // "while evaluating one one of the elements returned by the `operator`
// passed to builtins.genericClosure");
workSet.push_back(elem);
}
} catch (Error & err) {
err.addTrace(
nullptr,
"while calling %s on genericClosure element %s",
state.symbols[state.s.operator_],
ValuePrinter(state, *e, errorPrintOptions));
throw;
}
}