31 lines
1.5 KiB
Markdown
31 lines
1.5 KiB
Markdown
Debugging
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=========
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Some notes on how to debug Lean, which may also be applicable to debugging Lean programs in general.
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## Tracing
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In `CoreM` and derived monads, we use `trace![traceCls] "msg with {interpolations}"` to fill the structured trace viewable with `set_option trace.traceCls true`.
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New trace classes have to be registered using `registerTraceClass` first.
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Notable trace classes:
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* `Elab.command`/`Elab.step`: command/term macro expansion/elaboration steps
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Useful options modifying these traces for debugging syntax trees:
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```
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set_option pp.raw true
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set_option pp.raw.maxDepth 10
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```
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* `Meta.synthInstance`: typeclass resolution
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* `Meta.isDefEq`: unification
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* `interpreter`: full execution trace of the interpreter. Only available in debug builds.
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In pure contexts, one can instead use `dbgTrace! "msg with {interpolations}" val`, which will print the message directly to stderr before evaluating val. `dbgTraceVal val` can be used as a shorthand for `dbgTrace! "{val}" val`.
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Note that if the return value is not actually used, the trace code is silently dropped as well.
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## Debuggers
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`gdb`/`lldb` can be used to inspect stack traces of compiled Lean code, though they cannot print values of Lean variables and terms in any legible way yet.
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For example, `b lean_panic_fn` can be used to look at the stack trace of a panic.
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The [`rr` reverse debugger](https://github.com/rr-debugger/rr) is an amazing tool for investigating e.g. segfaults from reference counting errors, though better hope you will never need it...
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