diff options
Diffstat (limited to 'src/libutil/error.cc')
| -rw-r--r-- | src/libutil/error.cc | 122 |
1 files changed, 117 insertions, 5 deletions
diff --git a/src/libutil/error.cc b/src/libutil/error.cc index 1a1aecea5..e4f0d4677 100644 --- a/src/libutil/error.cc +++ b/src/libutil/error.cc @@ -9,9 +9,9 @@ namespace nix { const std::string nativeSystem = SYSTEM; -void BaseError::addTrace(std::shared_ptr<AbstractPos> && e, hintformat hint) +void BaseError::addTrace(std::shared_ptr<AbstractPos> && e, hintformat hint, bool frame) { - err.traces.push_front(Trace { .pos = std::move(e), .hint = hint }); + err.traces.push_front(Trace { .pos = std::move(e), .hint = hint, .frame = frame }); } // c++ std::exception descendants must have a 'const char* what()' function. @@ -200,13 +200,125 @@ std::ostream & showErrorInfo(std::ostream & out, const ErrorInfo & einfo, bool s auto noSource = ANSI_ITALIC " (source not available)" ANSI_NORMAL "\n"; - // traces - if (showTrace && !einfo.traces.empty()) { + /* + * Traces + * ------ + * + * The semantics of traces is a bit weird. We have only one option to + * print them and to make them verbose (--show-trace). In the code they + * are always collected, but they are not printed by default. The code + * also collects more traces when the option is on. This means that there + * is no way to print the simplified traces at all. + * + * I (layus) designed the code to attach positions to a restricted set of + * messages. This means that we have a lot of traces with no position at + * all, including most of the base error messages. For example "type + * error: found a string while a set was expected" has no position, but + * will come with several traces detailing it's precise relation to the + * closest know position. This makes erroring without printing traces + * quite useless. + * + * This is why I introduced the idea to always print a few traces on + * error. The number 3 is quite arbitrary, and was selected so as not to + * clutter the console on error. For the same reason, a trace with an + * error position takes more space, and counts as two traces towards the + * limit. + * + * The rest is truncated, unless --show-trace is passed. This preserves + * the same bad semantics of --show-trace to both show the trace and + * augment it with new data. Not too sure what is the best course of + * action. + * + * The issue is that it is fundamentally hard to provide a trace for a + * lazy language. The trace will only cover the current spine of the + * evaluation, missing things that have been evaluated before. For + * example, most type errors are hard to inspect because there is not + * trace for the faulty value. These errors should really print the faulty + * value itself. + * + * In function calls, the --show-trace flag triggers extra traces for each + * function invocation. These work as scopes, allowing to follow the + * current spine of the evaluation graph. Without that flag, the error + * trace should restrict itself to a restricted prefix of that trace, + * until the first scope. If we ever get to such a precise error + * reporting, there would be no need to add an arbitrary limit here. We + * could always print the full trace, and it would just be small without + * the flag. + * + * One idea I had is for XxxError.addTrace() to perform nothing if one + * scope has already been traced. Alternatively, we could stop here when + * we encounter such a scope instead of after an arbitrary number of + * traces. This however requires to augment traces with the notion of + * "scope". + * + * This is particularly visible in code like evalAttrs(...) where we have + * to make a decision between the two following options. + * + * ``` long traces + * inline void EvalState::evalAttrs(Env & env, Expr * e, Value & v, const Pos & pos, std::string_view errorCtx) + * { + * try { + * e->eval(*this, env, v); + * if (v.type() != nAttrs) + * throwTypeError("value is %1% while a set was expected", v); + * } catch (Error & e) { + * e.addTrace(pos, errorCtx); + * throw; + * } + * } + * ``` + * + * ``` short traces + * inline void EvalState::evalAttrs(Env & env, Expr * e, Value & v, const Pos & pos, std::string_view errorCtx) + * { + * e->eval(*this, env, v); + * try { + * if (v.type() != nAttrs) + * throwTypeError("value is %1% while a set was expected", v); + * } catch (Error & e) { + * e.addTrace(pos, errorCtx); + * throw; + * } + * } + * ``` + * + * The second example can be rewritten more concisely, but kept in this + * form to highlight the symmetry. The first option adds more information, + * because whatever caused an error down the line, in the generic eval + * function, will get annotated with the code location that uses and + * required it. The second option is less verbose, but does not provide + * any context at all as to where and why a failing value was required. + * + * Scopes would fix that, by adding context only when --show-trace is + * passed, and keeping the trace terse otherwise. + * + */ + + // Enough indent to align with with the `... ` + // prepended to each element of the trace + auto ellipsisIndent = " "; + + bool frameOnly = false; + if (!einfo.traces.empty()) { + size_t count = 0; for (const auto & trace : einfo.traces) { + if (!showTrace && count > 3) { + oss << "\n" << ANSI_WARNING "(stack trace truncated; use '--show-trace' to show the full trace)" ANSI_NORMAL << "\n"; + break; + } + + if (trace.hint.str().empty()) continue; + if (frameOnly && !trace.frame) continue; + + count++; + frameOnly = trace.frame; + oss << "\n" << "… " << trace.hint.str() << "\n"; if (trace.pos) { - oss << "\n" << ANSI_BLUE << "at " ANSI_WARNING << *trace.pos << ANSI_NORMAL << ":"; + count++; + + oss << "\n" << ellipsisIndent << ANSI_BLUE << "at " ANSI_WARNING << *trace.pos << ANSI_NORMAL << ":"; if (auto loc = trace.pos->getCodeLines()) { oss << "\n"; |