Expect exception with inline test

For some reason, Read World OCaml does not cover testing for exceptions.

https://dev.realworldocaml.org/testing.html

Not possible? I could reg exp the stack trace, I guess, but seems like a roundabout kinda way.

Seems like OUnit supports it? So inline test is inferior? tdd - Using ocaml OUnit2 assert_raise - Stack Overflow

One way to do it:

    try 
        Infer.run ast
    with
         ex -> [%test_eq exn] ex Type_error

But:

146 | ex → [%test_eq exn] ex Type_error
Error: PTyp expected

And there’s a test_eq_exn in this file: learn-ocaml/test_lib.ml at master · ocaml-sf/learn-ocaml · GitHub

My current solution:

Use let%test ...

And then

    try 
        ignore (Infer.run ast);
        false
    with
         | Infer.Type_error s -> true
         | _ -> false

Sad thing is, I won’t get any debug output at false, here, only

File “lib/test.ml”, line 134, characters 0-393: trivial concat type error is false.

You could shorten this a bit more with:

   match Infer.run ast with
         | exception Infer.Type_error s -> true
         | exception _  | _ -> false
1 Like

If you want to just test whether an expression raises, you can use:

let%test _ = Exn.does_raise (fun () -> Infer.run ast)

I would actually recommend using an “expect test” instead. You can use something like:

open Expect_test_helpers_base

let%expect_test _ =
  require_does_raise [%here] (fun () -> Infer.run ast);
  [%expect {||}]
;;

And the expect test will be filled in with the contents of the exception, or an error message if no exception was raised. This is better if you want to check which exception was raised.

3 Likes

Great advice, guys, will test. :heart:

Hm, how to install this? opam search gives nothing.

Trying opam install expect_test_helpers_kernel but:

===== ∗ 28   ↘ 21 =====

Yikes.

Hm, works better now but still getting

771 |     require_does_raise [%here] (fun () ->
Error: Uninterpreted extension 'here'.

[%here] is provided by ppx_here, one of the Jane Street suite of ppx rewriters. You can specify it individually in the dune file, or I usually use ppx_jane (which basically packages all of the JS ppx rewriters together):

(library
 (name ...)
 (preprocess (pps ppx_jane)))
1 Like