如何强制OCaml推断更通用的类型? [英] how to force OCaml to infer a more general type?

问题描述

我想定义一个函数，该函数接受一个可选参数，该参数是一个函数('a->'b).默认值应该是标识，实际上是('a -> 'a)，但是我看不出为什么它不应该与更通用的('a -> 'b)兼容.当我尝试时:

I want to define a function that accepts an optional argument which is a function ('a -> 'b). The default value should be the identity, which is actually ('a -> 'a), but i see no reason why it should not be compatible with the more general ('a -> 'b). When i try:

let optional_apply ?f i =
    match f with 
    | None -> i + 4
    | Some fn -> fn (i + 4)

我总是得到窄型?f:(int -> int) -> int -> int.但我想将f保留为int -> 'b.我能做些什么?还是因为optional_apply没有确定的类型，这才是不合理的?如果是这样，我将如何获得类似的功能?

I always get the narrow type ?f:(int -> int) -> int -> int. But I want to keep f as int -> 'b. What can i do? Or is this just unsound, since optional_apply would not have a definite type? If so, how would I get a similar functionality?

推荐答案

不可能，f参数不能为可选.一个简单的解释是，可选参数只是语法糖. 因此，不用加糖，您的函数可以按以下形式重写:

It is impossible, the f argument must not be optional. A simple explanation, is that optional parameters are just a syntactic sugar. So without sugar your function can be rewritten in a following form:

let optional_apply f n = match f with
  | Some f -> f (n + 4)
  | None -> (n + 4)

在这里，类型检查器仅允许我们为f使用一种类型:int -> int.如果它允许我们使用int -> 'a类型，则表达式的None路径将不正确.换句话说，根据f是None还是Some，optional_apply将得出不同的类型.这被认为是不健全的.

And here typechecker allows us only one type for f: int -> int. If it allowed us an int -> 'a type, then the None path of expression would be unsound. In other words, depending on whether f is None or Some the optional_apply will evaluate to different types. And this is considered unsound.

证明不健全的最好方法是举一个简单的例子，其中typechecker将允许不健全的程序:

The best way to prove the unsoundness is to give a simple example where typechecker will allow unsound program:

let f = ref None
let g n = optional_apply ?f:!f n

使用此定义，如果类型检查器允许f参数保持多态，那么我们可以随时通过将f引用更改为任何其他内容来破坏" g功能.

with this definitions, if type checker allowed f parameter to remain polymorphic, then we can at any time "break" g function by changing f reference to anything else.

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