f# 为多种类型和重载运算符定义通用 function

Question

I'm trying to define an inline function that can perform arithmetic operations on multiple types - including on user defined types - in a generic way. However it seems like the inline function seems to infer it's type prematurely.

My custom type is the following:

type ^T Foo (x: ^T) =
    static member inline (*) (x: ^T, y: Foo< ^T>) = Foo x

and here is how I want to use it:

let foo = Foo 1

let F () = 1 * foo // works
let inline G x = 1 * x // G inferred as int -> int

let z1 = G 1 // works
let z2 = G foo // error: expected int not Foo

What is the best way to write generic functions like G while keeping it simple?

Answer 1

The problem is that F# type inference uses weak resolution here:

let inline G x = 1 * x // G inferred as int -> int

That is, in presence of a binary operator it uses the evidence that one operator is a known type to match the other one with the same type.

This behavior might change in future versions, in presence of the inline keyword.

For the moment the standard way write your function is using a generic one:

let inline G x = LanguagePrimitives.GenericOne * x