When I first read about the time-travel feature of the ocaml debugger I was very intrigued but never got around to trying it out in practice at work. This weekend I decided to give it a go.


by Mads Hartmann - 15 Nov 2014

I wanted to try it out on a simple but non-trivial program (in terms of setup, i.e. a program that requires opam packages to compile) so I used the hello world example of the ocaml-cohttp library.


Before the ocamldebugger can work with your programs they need to be compiled with the -g flag enabled. This is also required for any libraries that you use. You can configure the ocamlc parameters that opam uses by setting OCAMLPARAM; in this case you want to set it to _,g. You will want to run opam reinstall on any package you have previously installed to make sure they get recompiled.

The second thing you need to do is tell ocamldebug where to look for your compiled files. Unfortunately this is not as easy as it is with utop. Luckily I came across this stack overflow thread which explains how to do it. I ended up using a slightly different approach as you can’t pass arguments to ocamldebug when running it from within Emacs (at least I couldn’t get it to work). I decided to write the results of ocamlfind query -recursive core async cohttp cohttp.async into a file named .ocamldebug and prefix each line with directory. After starting the debugger in Emacs you then need to run source <PATH> to configure it.

Using the debugger

Starting the debugger from within Emacs is just a matter of running M-x ocamldebug and point it to the executable you want to debug.

You start the program by writing run in the ocamldebug buffer. You set break-points in your code using C-x C-a C-b and step through the code use C-c C-s. You can inspect values using C-x C-a C-p. For more information read this chapter of the OCaml Users Guide.


Unfortunately I found a couple of shortcomings when I played around with the debugger for a bit.

No support for -pack‘ed compilation units

Once you start setting some breakpoints and step through the code you’re very likely to get an error like the following.

(ocd) step
Time: 1084742 - pc: 4416712 - module Cohttp.Request
No source file for Cohttp.Request.

This error had me puzzled for a bit as I thought I had already told ocamldebug where to look for my opam packages. I joined the #ocaml IRC channel and whitequark was able to come up with a potential explanation. It seems that ocamldebug doesn’t support compilation units that contain submodules that have been added using -pack and -for-pack. Search for -pack in this section for the OCaml Users Guide for more information.

No arbitrary OCaml code execution

Once you hit a breakpoint it’s possible to inspect the values of the variables in the current scope. This is great, but it would have been even better if you could execute arbitrary OCaml code in the current scope. This doesn’t seem to be possible. To be fair this might be better classified as an awesome feature rather than a shortcoming.