F# unit testing and code coverage

And you thought it was going to be easy to get full coverage of functional code... Try this little example, assuming a Haskell-like Either<'a,'b>:

	let ExpandFile name =
	try
	Right (new FileInfo(name)).FullName
	with
	| :? ArgumentException -> Left "fileNotFound"

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Feed it "dummy.txt" for the happy case, and something like "!!" for the bad case -- and you have an uncovered code-point for the untaken branch of the pattern -- i.e. rethrow any other exception type.

Then take that second case

	[<Test>]
	member self.ExpandFileWithJunkShouldFail() =
	let expected = MakeEither "fileNotFound" String.Empty false
	Assert.That( ExpandFile "!!", Is.EqualTo( expected ), "should not be a file name") |> ignore

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from which NCover reveals we have hidden classes like <StartupCode$Tinesware-InfrastructureTests>.$Tests+ExpandFileWithJunkShouldFail@24 and <StartupCode$Tinesware-InfrastructureTests>.$Tests+ExpandFileWithJunkShouldFail@24-1 where the module is Tinesware.InfrastructureTests, the file Tests.fs and the Assert call is on line 24 of the file. And the first is 100% covered and the second is 0% covered.

The Feb 2010 CTP only generates 1 indirecting class on that line, according to Reflector, the @24 one and not the @24-1 one.