Module Ppx_dbg

Extension point rewriter for debug trace logging

This ppx rewriter reads a cookie to determine whether to rewrite in "debug" mode or not. To enable "debug" mode, pass --cookie 'ppx_dbg_enabled="1"' (or with true instead or 1).

It rewrites [%dbg] ~call ~retn ~rais to a call Dbg.dbg ~call ~retn ~rais fun_name where fun_name is the value of __FUNCTION__ at the call site. This is only done in debug mode, otherwise [%dbg] ~call ~retn ~rais is rewritten to (fun k -> k ()).

Similarly, it rewrites [%dbgs] ~call ~retn ~rais to a call Dbg.dbgs ~call ~retn ~rais fun_name. This is only done in debug mode, otherwise [%dbgs] ~call ~retn ~rais is rewritten to (fun x -> x).

Similarly, [%Dbg.info], [%Dbg.infok], [%Dbg.printf], [%Dbg.fprintf], [%Dbg.kprintf], and [%Dbg.call] are rewritten to their analogues in the Dbg module, or (); and [%Dbg.retn] is rewritten to a call to Dbg.retn or (fun x -> x).

For example, this enables writing

let func arg =
  [%dbg]
    ~call:(fun {pf} -> pf "@ %a" pp_arg_type arg)
    ~retn:(fun {pf} -> pf "%a" pp_result_type)
  @@ fun () -> func arg

or

let func arg =
  [%Dbg.call fun {pf} -> pf "@ %a" pp_arg_type arg]
  ;
  func arg
  |>
  [%Dbg.retn fun {pf} -> pf "%a" pp_result_type]

to trace calls to func in debug mode while completely compiling out the debug code in non-debug builds.

This mechanism can also be used e.g. to dynamically check assertions only in debug mode.

Additionally, [%debug] is rewritten to the compile-time boolean constant indicating if rewriting was done in debug mode.