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Problem with constraint on type parameter
f#compiler

Consider the following code, written as part of an NUnit test:

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let expect_throw<'a> f =
  try
    f()
    Assert.Fail("should throw")
    ""
  with
    :? 'a as e -> e.Message

With this code I get the error
Type constraint mismatch. The type 'a is not compatible with the type exn.
A type parameter is missing a constraint 'when 'a :> exn'
Yet when I try to add the constraint, replacing <'a> with <'a when 'a :> exn>, I get the error
The code is not sufficiently generic. The type variable #exn could not be generalized
because it would escape its scope.
I have no idea what this message means. So how do I define a function with a constrained type parameter?
By the way, this code was translated from the following C# code:

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public delegate void Block();
public string ExpectThrow<E>(Block block) where E : Exception
{
    try
    {
        block();
        Assert.Fail("should throw");
        return null; // make compiler happy
    }
    catch (E e)
    {
        return e.Message;
    }
}
.

Hi, it works fine here -
I tried it in the latest version of FSI (F# 1.9.3.14) and the output is following:

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> #light
- let expect_throw<'a when 'a :> exn> f =
-   try
-     f()
-     // Assert.Fail("should throw")
-     "fail"
-   with
-     :? 'a as e -> e.Message
- ;;

val expect_throw : (unit -> unit) -> string

What version are you using? Do you use FSI or FSC? Can you try it using the latest version?

By on 3/20/2008 4:05 PM ()

I'm using version 1.9.3.14. I just now tried it with fsi and it

worked, but when compiled with fsc (via Visual Studio 2005) it gives me

the error.

By the way, I'm running into a similar situation with the following code snippet:

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  async { do raise e in return 0 }

fsi accepts this, but fsc does not. With fsc I have to write

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async { do raise e
           return 0 }
By on 3/20/2008 4:28 PM ()

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