slamko/ppx_catch

Ocaml PPX for no-exception error handling

Simple PPX rewriter to catch exceptions and wrap the result into (‘a, string) result

Overview

May be useful to handle the case when the function inside List.map fails:

let%catch func lst =
  List.map (fun x ->
    if x > 0 then failwith "Some problem" else x + 1) lst 

becomes:

let%catch func lst =
  try
    Ok (List.map (fun x ->
      if x > 0 then failwith "Some problem" else x + 1) lst)
  with
  | Failure err -> Error (*[ "func: " + err ]*) (* error messages with tracing *)
    (* and so on *) ...
  | _ -> Error "func: Unknown error" 

Extension handles both top-level and “embedded” let bindings:

let foo =
  let%catch wants_to_raise () = (* wrap some Stdlib function for example *)
    (* some logic *)
    failwith "Stdlib exception" (* call to Stdlib function that raises an exception *)
  in

  wants_to_raise () (* returns ('a, string) result *)

Also works with “function” syntax:

let%catch wrap_function = function
  | Some res -> res
  | None -> failwith "function syntax works too"

It basically wraps each case of the pattern matching.

let () =
  match wrap_function None with
  | Ok ok -> Printf.printf "Ok\n" ;
  | Error err -> Printf.printf "%s\n" err ;
  () 

prints :

wrap_function: function syntax works too

Quickstart

To instal this rewriter as opam package:

git clone https://github.com/slamko/ppx_catch.git &&
cd ppx_catch &&
dune build &&
opam install .

Now to use it in your project add ppx_catch to library list and tell to use it as a preprocessor in your dune file:

(libraries
            ppx_catch)
(preprocess (pps ppx_catch))

For a complete dune file example see test/dune

LICENSE

GPL-v3.0