Parametric Polymorphism vs Subtype polymorphism F#

Question

What is the difference (if any) between these two F# type signatures?

UseTheStream<'a when 'a :> Stream> : 'a -> unit

and

UseTheStream : (stream : Stream) -> unit

Do they mean the same thing in this case?

msdn says the following about the (:>) Type Constraint

type-parameter :> type --   The provided type must be equal to or derived from the type      specified, or, if the type is an interface, the provided type must implement the interface.

This would indicate that the two signatures are saying the same thing. So Functionally, how are they different?

Answer 1

They are different. Most importantly, the first function is generic. In your example it probably doesn't matter, but if the type parameter affects the function's return type, it does:

let UseTheStream (stream: #Stream) = stream
let UseTheStreamStrict (stream: Stream) = stream

let s1 = new MemoryStream() |> UseTheStream
let s2 = new MemoryStream() |> UseTheStreamStrict

s1 is MemoryStream. s2 is Stream.

NOTE: #T is shorthand for 'U when 'U :> T.