ppx_log_calls is a tool that helps OCaml developers debugging their programs. It is a ppx deriver that adds logging to any function whose definition is annotated by a %log extension point. Any call to a function with such an annotation is logged at execution time, including any parameter value for which an explicit type anotation is given.

Check out ppx_minidebug for a potentially more mature project offering similar functionality.

let%log f ?(x : int option) (y : bool) z = ignore (x, y, z)

let () = f true "z"

The program above leads to the following console output:

f ?x:None true _

Note that the value "z" is not printed since no explicit type is given for the parameter z.

If you add a type annotation for the result like so:

let%log f ?(x : int option) (y : bool) z : unit = ignore (x, y, z)

Then in addition to the previous console output (before the execution of the function) the following output is displayed (after its execution):

f ?x:None true _ = ()

When using dune it suffices to add the following stanzas to your project's dune file:

(preprocess (pps ppx_deriving.show ppx_log_calls))
(libraries ppx_log_calls.runtime)

Then simply add a %log annotation to any let-binding for the defined function(s) to be logged. In order for the supplied arguments to be displayed as well an explicit type annotation have to be supplied as well as a pretty-printing function for that type.

If you write for example :

let%log f (x : mytype) = ignore x

Then there should be a pretty-printing function with the name pp_mytype in scope. If the type name is t then the pretty-printer is simply called pp.

Note that this naming scheme is compatible with the naming of pretty-printing functions generated by ppx_deriving.show.

Look at test/basic.ml for a simple example. It can be executed by running dune test -f.

ppx_log_calls is distributed under the terms of the BSD3 license (see LICENSE file).