How can I pass a constructor as an arg and then pattern match on it? - ocaml

Question

e.g.
let expect_raises f exc =
try f ()
with
| exc -> "Expected error raised"
| e -> "Unexpected error raised"
in
expect_raises (fun () -> raise Not_found) Not_found (* "Expected" *)
expect_raises (fun () -> raise Invalid_argument "bad") Not_found (* "Unexpected" *)
But this doesn't work because I can't pattern match on the exc arg, it just becomes the pattern variable.
Is there some way to do this?

Answer 1

Pattern matching in OCaml works essentially against constants (or more generally trees of constants with wildcard slots). So there's no way to use pattern matching to do what you want. This is the same for exceptions as it would be if you (for example) wanted to pass an integer value.
Similarly to integers, you can compare exception values for equality. So you can write your function like this:
let expect_raises f exc =
try f ()
with e ->
if e = exc then "Expected error raise"
else "Unexpected error raised"
Here are the test cases:
# expect_raises (fun () -> raise Not_found) Not_found;;
- : string = "Expected error raise"
# expect_raises (fun () -> raise (Invalid_argument "bad")) Not_found;;
- : string = "Unexpected error raised"
However, this seems like a fragile test because the equality comparison depends on the exact contents of the exception constructor. The compiler warns (reasonably so IMHO) against doing this:
# match (try failwith "abc" with e -> e) with
| Failure "abc" -> 20
| _ -> 30;;
Warning 52 [fragile-literal-pattern]: Code should not
depend on the actual values of this constructor's
arguments. They are only for information and may change
in future versions. (See manual section 11.5)

Answer 2

FWIW my solution at the moment looks like:
let expect_raises f exc_f pp =
let result_opt =
try exc_f f
with e -> Some (Error e)
in
match result_opt with
| Some (Ok a) ->
fail ## Format.asprintf "Unexpected result: %a" pp a
| Some (Error e) ->
fail ## Printf.sprintf "Unexpected error: %s" (Printexc.to_string e)
| None -> pass (* the correct result *)
let test_fun () ->
let exc_f f = try Some (Ok (f ())) with Not_found -> None in
expect_raises (fun () -> funtotest 123) exc_f pp
let test_other_fun () ->
let exc_f f = try Some (Ok (f ())) with Invalid_argument _ -> None in
expect_raises (fun () -> otherfuntotest "abc") exc_f pp
Having to define the exf_f is a bit cumbersome - I would love to be able to factor out the common part leaving just the exception type. But at least this works and allows to match by constructor alone and not by value.
I guess it would be possible to make a ppx rewriter to replace manual definition of exc_f func with something like:
expect_raises (fun () -> otherfuntotest "abc") [%excf Invalid_argument] pp