In F#, How can I cast a function to a different delegate?

Question

I need to cast a function with the signature Thing -> Thing -> int to a Comparison<Thing> .

For example, when I have:

Array.Sort(values, mySort)

I get an error stating " This expression was expected to have type Comparison but here has type Thing -> Thing -> int "

When I try this:

Array.Sort(values, (fun (a, b) -> mySort a b))

(actually, this is by way of a wrapper object)

I get an error stating " Type constraint mismatch. The type ''a -> 'b' is not compatible with the type 'Comparison' "

How am I supposed to provide a Comparison<T> ?

FYI: Comparison<T> is a delegate with the signature 'T * 'T -> int

Answer 1

This works for me:

let values = [|1;2;3|]
let mySort a b = 0
Array.Sort(values, mySort)

But have you tried:

Array.Sort(values, Comparison(mySort))

Answer 2

Oddly, despite the fact that delegate signatures are expressed in tupled form (eg 'T * 'T -> int ), you need to provide your function in curried form (eg fun ab -> mySort ab ). See section 6.4.2 of the spec.