Check if throws

[cmeeren]

Is it possible to check if a function throws an exception without having try ... with in the property and explicitly returning true or false?

[moodmosaic]

Would it be possible to provide an example?

[cmeeren]

This works, but feels rather heavy-handed:

[<Fact>]
let ``throws if hour is < 0 or > 23`` () =
  Property.check <| property {
    let! dt = Gen.dateTime
    let! hr = Range.linearBounded() |> Gen.int |> Gen.filter (fun hr -> hr < 0 || hr > 23)
    try
      nextOClock hr dt |> ignore
      return false
    with
      :? ArgumentException -> return true
  }

[cmeeren]

Ideally there'd be some kind of Property.checkThrows<'exn> that I could use. For example:

[<Fact>]
let ``throws if hour is < 0 or > 23`` () =
  Property.checkThrows<ArgumentException> <| property {
    let! dt = Gen.dateTime
    let! hr = Range.linearBounded() |> Gen.int |> Gen.filter (fun hr -> hr < 0 || hr > 23)
    nextOClock hr dt

[moodmosaic]

What happens when you replace Property.checkThrows<ArgumentException> with Property.check?

[moodmosaic]

Oh, now I see what you mean; you want the exception to be thrown, and in this case, the property to pass.

[cmeeren]

👍

[moodmosaic]

@cmeeren, we recently added an internal function which does what you describe:

fsharp-hedgehog/src/Hedgehog/Property.fs

Lines 382 to 389 in 70d4454

	/// Converts a possibly-throwing function to
	/// a property by treating "no exception" as success.
	let internal fromThrowing (f : 'a -> unit) (x : 'a) : Property<unit> =
	try
	f x
	success ()
	with
	| _ -> failure

AFAICT, this function does exactly what you need. If so, we can make it public in the next release.

[cmeeren]

No, that does the opposite. It treats throws as failures. I want to test that a function does indeed throw an exception (the test should fail if nothing is thrown). Furthermore, it should check the type of exception (the test should fail if the wrong exception is thrown). And ideally, the user should be able to choose whether it needs to match the exception exactly or if a subclass of the specified exception is fine.

IIRC FsCheck does this nicely.

[moodmosaic]

OK, then it's the opposite of that function plus pattern matching on the type of exception.

[cmeeren]

Yes. See my example above for a suggestion of a nice and short syntax:

Property.checkThrows<ArgumentException>

I suggest that the default behavior is that the test passes if ArgumentException or any subclass of ArgumentException is thrown, and fails otherwise. If the user wants the test to pass for only ArgumentException (not a subclass), they can use another function:

Property.checkThrowsExact<ArgumentException>