How to debug OCaml dune test program?

Question

Let's say I:

dune init proj foobar
cd foobar

Then modify test/test_foobar.ml to be:

let () = print_endline "First"
let () = print_endline "Second"
let () = print_endline "Third"

I can execute the program with dune test.
How can I run the program and step each line individually in a debugger with source available? Either ocamldebug or gdb is probably fine.

I can do gdb --tui --args ./_build/default/test/test_foobar.exe but it does NOT show the source.

I don't see any suitable files to launch with ocamldebug e.g.:

$ ocamldebug ./_build/default/test/test_foobar.ml
 OCaml Debugger version 5.2.0
(ocd) r
Loading program... < shortened >/foobar/./_build/default/test/test_foobar.ml is not a bytecode file.

Working with OCaml 5.2.0 and dune 3.15.3

• Chris: Does this answer your question? Debugging with dune
• Costava: @Chris That is some good info but the sticking point for me was that gdb does not show the OCaml source until a breakpoint is hit. gdb will confusingly run all the way to the end of the program and show "No Source Available" the whole time. I am used to gdb's behavior with C programs where the source is shown immediately before the program even starts.

Answer 1

Add (modes byte exe) to your test/dune file to cause a byte-code version to be built also e.g. _build/default/test/test_foobar.bc which can be run in ocamldebug.

The test/dune file:

(test
 (name test_foobar)
 (modes byte exe)
)

---

gdb does run to the end of the program while showing "No Source Available" the whole time.
If you set a breakpoint and hit it though, the source will appear.

e.g.

(gdb) break test/test_foobar.ml:1