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libexpr: Support structured error classes

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While preparing PRs like #9753, I've had to change error messages in
dozens of code paths. It would be nice if instead of

    EvalError("expected 'boolean' but found '%1%'", showType(v))

we could write

    TypeError(v, "boolean")

or similar. Then, changing the error message could be a mechanical
refactor with the compiler pointing out places the constructor needs to
be changed, rather than the error-prone process of grepping through the
codebase. Structured errors would also help prevent the "same" error
from having multiple slightly different messages, and could be a first
step towards error codes / an error index.

This PR reworks the exception infrastructure in `libexpr` to
support exception types with different constructor signatures than
`BaseError`. Actually refactoring the exceptions to use structured data
will come in a future PR (this one is big enough already, as it has to
touch every exception in `libexpr`).

The core design is in `eval-error.hh`. Generally, errors like this:

    state.error("'%s' is not a string", getAttrPathStr())
      .debugThrow<TypeError>()

are transformed like this:

    state.error<TypeError>("'%s' is not a string", getAttrPathStr())
      .debugThrow()

The type annotation has moved from `ErrorBuilder::debugThrow` to
`EvalState::error`.
This commit is contained in:
Rebecca Turner 2024-01-22 17:08:29 -08:00
parent c62c21e29a
commit c6a89c1a16
No known key found for this signature in database
40 changed files with 653 additions and 545 deletions
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18
src/libexpr/value-to-json.cc
View file

@ -80,7 +80,7 @@ json printValueAsJSON(EvalState & state, bool strict,
try {
out.push_back(printValueAsJSON(state, strict, *elem, pos, context, copyToStore));
} catch (Error & e) {
e.addTrace({},
e.addTrace(state.positions[pos],
hintfmt("while evaluating list element at index %1%", i));
throw;
}
@ -99,13 +99,12 @@ json printValueAsJSON(EvalState & state, bool strict,
case nThunk:
case nFunction:
auto e = TypeError({
.msg = hintfmt("cannot convert %1% to JSON", showType(v)),
.errPos = state.positions[v.determinePos(pos)]
});
e.addTrace(state.positions[pos], hintfmt("message for the trace"));
state.debugThrowLastTrace(e);
throw e;
state.error<TypeError>(
"cannot convert %1% to JSON",
showType(v)
)
.atPos(v.determinePos(pos))
.debugThrow();
}
return out;
}
@ -119,7 +118,8 @@ void printValueAsJSON(EvalState & state, bool strict,
json ExternalValueBase::printValueAsJSON(EvalState & state, bool strict,
NixStringContext & context, bool copyToStore) const
{
state.debugThrowLastTrace(TypeError("cannot convert %1% to JSON", showType()));
state.error<TypeError>("cannot convert %1% to JSON", showType())
.debugThrow();
}