Why does F# using Xunit require type information when Asserting equality on Strings

Question

I'm using F# and Xunit. (I'm relatively new to both)

I've found that when I use Xunit's Assert.Equal() I need to specify "<string>" when the types being compared are string.

For example this run's and compiles:

[<Fact>]
let Test_XunitStringAssertion() =
    let s1 = "Stuff"
    Assert.Equal<string>("Stuff",s1)

My question is, why can't I remove "<string>" and just assert "Assert.Equal("Stuff",s1)" instead?

It looks to me like the compiler knows the types of both arguments, so why the fuss?

Here are the errors returned when compiling Assert.Equal("Stuff",s1):

error FS0041: A unique overload for method 'Equal' could not be determined based on type information prior to this program point. The available overloads are shown below (or in the Error List window). A type annotation may be needed.
error FS0041: Possible overload: 'Assert.Equal<'T>(expected: 'T, actual: 'T) : unit'.
error FS0041: Possible overload: 'Assert.Equal<'T>(expected: seq<'T>, actual: seq<'T>) : unit'.
error FS0041: Possible overload: 'Assert.Equal<'T>(expected: 'T, actual: 'T, comparer: System.Collections.Generic.IEqualityComparer<'T>) : unit'.
error FS0041: Possible overload: 'Assert.Equal(expected: float, actual: float, precision: int) : unit'.
error FS0041: Possible overload: 'Assert.Equal(expected: decimal, actual: decimal, precision: int) : unit'.
error FS0041: Possible overload: 'Assert.Equal<'T>(expected: seq<'T>, actual: seq<'T>, comparer: System.Collections.Generic.IEqualityComparer<'T>) : unit'.

Answer 1

That's because string can be matched by both the first and second overloads (remember: string :> seq<char>).

Answer 2

Your example with <string> removed runs without error for me as I'd expect (although string :> seq<char> as @Ramon Snir points out, the overload resolution algorithm resolves the ambiguity by recognizing that the supplied string types are "closer" to string than seq<char>).

[<Fact>]
let Test_XunitStringAssertion() =
    let s1 = "Stuff"
    Assert.Equal("Stuff",s1)

I guess the sample you provided is not exactly the same as the real code which is causing you problems. Maybe s1 in your real code is not actually a string (or at least the compiler doesn't know it is).