F#:算术运算符和多态性损失(值限制?) [英] F#: arithmetic operator and loss of polymorphism (value restriction?)

问题描述

此代码无法编译:

let f = fun x y -> x <<< y // bit shift
let g = fun x y -> x <<< y

[<EntryPoint>]
let main _ =
  printfn "%d" <| f 1 10
  printfn "%d" <| f 1L 10 // error
  printfn "%d" <| g 1L 10
  0

(7,21): error FS0001: This expression was expected to have type
    int
but here has type
    int64

• http://ideone.com/qktsOb
• http://ideone.com/qktsOb

我猜统一器在看到它们第一次出现时就修复了与f和g关联的类型参数.什么决定了这个过程?我认为这与值限制"非常相似，但是f和g已经被eta扩展了！这是一个难题.

I guess the unifier fixed the type parameters associated with f and g upon seeing their first occurrences. What governs this process? I think this is very similar to "value restriction" but f and g are already eta-expanded! This is a hard problem.

• http://en.wikipedia.org/wiki/Value_restriction

我当然可以想象，在预定义的运算符后面加上整数类型具有特殊的多态性，这会带来一些不可思议的魔力，但这只是我的猜测.任何信息表示赞赏.

I would surely imagine there's some black magic behind typing predefined operators with ad-hoc polymorphism over integral types, but that's just my speculation. Any information appreciated.

推荐答案

通用数字编程是使用静态成员约束完成的，该成员约束无法在.NET类型系统中表示.它们仅存在于F#中，因此必须标记为inline.

Generic numeric programming is done using static member constraints, which can't be represented in the .NET type system. They exist only in F# and therefore must be marked inline.

您的代码可以这样写:

let inline f x y = x <<< y // bit shift
let inline g z y = z <<< y

[<EntryPoint>]
let main _ =
  printfn "%d" <| f 1 10
  printfn "%d" <| f 1L 10 // works too
  printfn "%d" <| g 1L 10
  0

有关MSDN的更多信息:
内联函数
静态解析的类型参数

More info on MSDN:
Inline Functions
Statically Resolved Type Parameters

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