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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
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40 changed files with 653 additions and 545 deletions
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27
src/libutil/error.hh
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@ -84,9 +84,14 @@ inline bool operator>=(const Trace& lhs, const Trace& rhs);
struct ErrorInfo {
Verbosity level;
hintformat msg;
std::shared_ptr<Pos> errPos;
std::shared_ptr<Pos> pos;
std::list<Trace> traces;
/**
* Exit status.
*/
unsigned int status = 1;
Suggestions suggestions;
static std::optional<std::string> programName;
@ -103,18 +108,21 @@ class BaseError : public std::exception
protected:
mutable ErrorInfo err;
/**
* Cached formatted contents of `err.msg`.
*/
mutable std::optional<std::string> what_;
/**
* Format `err.msg` and set `what_` to the resulting value.
*/
const std::string & calcWhat() const;
public:
unsigned int status = 1; // exit status
BaseError(const BaseError &) = default;
template<typename... Args>
BaseError(unsigned int status, const Args & ... args)
: err { .level = lvlError, .msg = hintfmt(args...) }
, status(status)
: err { .level = lvlError, .msg = hintfmt(args...), .status = status }
{ }
template<typename... Args>
@ -149,6 +157,15 @@ public:
const std::string & msg() const { return calcWhat(); }
const ErrorInfo & info() const { calcWhat(); return err; }
void withExitStatus(unsigned int status)
{
err.status = status;
}
void atPos(std::shared_ptr<Pos> pos) {
err.pos = pos;
}
void pushTrace(Trace trace)
{
err.traces.push_front(trace);