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.
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
ocamlfind query -recursive core async cohttp cohttp.async into a
.ocamldebug and prefix each line with
starting the debugger in Emacs you then need to run
source <PATH> to
2 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
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.
3.1 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
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
-for-pack. Search for
-pack in this section for the OCaml Users
Guide for more information.
3.2 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.