Question

I am trying to write my first generic function in F#, and it looks like I am missing some concept here :

module Assert =
    let ThrowsException<'e : exn>  functionUnderTest  =
    try 
        let result = functionUnderTest
        printfn "fails"
    with 
    | :? 'e  ->  printfn  "succeeds"
    | _  -> printfn "fails"

stdin(18,28): error FS0010: Unexpected symbol ‘:’ in pattern. Expected ‘>’ or other token.

I would like to be able to use my function like that to test if an exception was raised.

let myFunction=HttpClient.postDocRaw "http://notexisting.com/post.php" "hello=data"
Assert.ThrowsException System.Net.WebException myFunction

I would like to achieve this tool for testing so if anybody has a solution , that’s nice, but furthermore, I am learning F#, and I would like to know where I go wrong trying to write functions like this.

final code :

module Assert =
    let ThrowsException<'e when 'e :> exn>  functionUnderTest  =
        try 
           let actual = functionUnderTest()
           printfn "succeeds"
        with 
        | :? 'e  ->  printfn  "succeeds"
        | _  ->  Assert.Fail()


[<TestClass>]
type HttpClientTest() = 
[<TestMethod>]
    member x.PostDataWrongUrl() = 
        Assert.ThrowsException<System.Net.WebException>  (fun() ->  HttpClient.postDocRaw "http://notexisting.com/post.php" "hello=data")

Answer 1

The error says “Expected ‘>'” because a type constraint takes the form 'T when 'T :> baseType. Your code should be:

module Assert =
  let ThrowsException<'e when 'e :> exn> functionUnderTest =
    try 
      let result = functionUnderTest
      printfn "fails"
    with 
      | :? 'e -> printfn  "succeeds"
      | _  -> printfn "fails"

See Constraints on MSDN for more info.