Some notes on how to debug Lean, which may also be applicable to debugging Lean programs in general.


In CoreM and derived monads, we use trace![traceCls] "msg with {interpolations}" to fill the structured trace viewable with set_option trace.traceCls true. New trace classes have to be registered using registerTraceClass first.

Notable trace classes:

  • Elab.command/Elab.step: command/term macro expansion/elaboration steps

    Useful options modifying these traces for debugging syntax trees:

    set_option pp.raw true
    set_option pp.raw.maxDepth 10
  • Meta.synthInstance: typeclass resolution

  • Meta.isDefEq: unification

  • interpreter: full execution trace of the interpreter. Only available in debug builds.

In pure contexts or when execution is aborted before the messages are finally printed, one can instead use the term dbg_trace "msg with {interpolations}"; val (; can also be replaced by a newline), which will print the message to stderr before evaluating val. dbgTraceVal val can be used as a shorthand for dbg_trace "{val}"; val. Note that if the return value is not actually used, the trace code is silently dropped as well. In the language server, stderr output is buffered and shown as messages after a command has been elaborated, unless the option server.stderrAsMessages is deactivated.


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. For example, b lean_panic_fn can be used to look at the stack trace of a panic.

The rr reverse debugger is an amazing tool for investigating e.g. segfaults from reference counting errors, though better hope you will never need it...