Type inference of functions as arguments

问题

I would like to write a function which takes several tupples as arguments and choose theirs ith elements and passes to another function, where i is given as another argument. I've tried sth like this:

let function (tup1:'A*'A) (tup2:'B*'B) i =
    otherFunction (i tup1) (i tup2)
function Tup1 Tup2 fst

I've got an error, because i was expected to be 'A*'A ->'A not 'B*'B->'B. Is it any way to make this code to work?

Thanks in advance.

回答1:

You basically want to pass an argument of type ∀'a.'a*'a->'a, but in F# (and other MLs), only rank-1 polymorphism is supported so you can't do this directly. The workaround is to define a new type with a generic method to emulate higher-rank polymorphism:

type Untupler =
    abstract Apply : 'a*'a -> 'a

let myFunction tup1 tup2 (i:Untupler) =
    otherFunction (i.Apply tup1) (i.Apply tup2)

myFunction Tup1 Tup2 { new Untupler with member __.Apply (x,y) = x }

回答2:

When you use the function i with tup1, it's inferred to be of the type 'A * 'A -> 'A. This means that when you use it with tup2, the only way that can work is if tup2 is also an 'A * 'A.

The function i can't change type within the same function. If there's a way to do what you want, I'm not aware of it, but you can sidestep the issue like this:

let myFunction x y =
    otherFunction x y

Call it like this:

myFunction (fst tup1) (fst tup2)

This puts slightly more responsibility of the caller, but the advantage is that it's possible :)

来源：https://stackoverflow.com/questions/32156122/type-inference-of-functions-as-arguments

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