问题
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?
回答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
.
来源:https://stackoverflow.com/questions/12642253/parametric-polymorphism-vs-subtype-polymorphism-f