Parametric Polymorphism vs Subtype polymorphism F#

笑着哭i 提交于 2019-12-06 06:11:53
Daniel

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.

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