types - 开放对象类型统一得太早

Question

我有一个处理大量计算的大型 let block ，我试图通过将它的 block 分解成函数来将它分解成更小的部分。我需要将一个对打开的对象进行操作的函数传递给这些组件，并且遇到了以下简化代码中最能表达的烦恼:

type 'a foo = 'a
  constraint 'a = < bar: string option; .. >

type foo' = < bar: string option; >

type bar = < bar: int option; qux: int option >

type ('a, 'b) fn = 'a foo -> 'b foo

let a = object method bar = None method qux = None end
let b = object method bar = None end

(* Doesn't work *)
let xyz fn =
  let res = fn a in
  let res' = fn b in
  res, res'

(* Doesn't work *)
let xyz (fn:('a, 'b) fn) =
  let res = fn a in
  let res' = fn b in
  res, res'

(* Doesn't work *)
type fn' = foo' -> foo'
let xyz (fn:fn') =
  let res = fn a in
  let res' = fn b in
  res, res'

(* Sub-par solution: imagine needing several of these conversions across several
   functions, which is the situation I have when trying to decompose a large let
   block into component functions like xyz. *)
let xyz (fn:('a, 'b) fn) =
  let res = fn (a :> foo') in
  let res' = fn b in
  res, res'

(* Another sub-par solution: *)
type ('a, 'b) fn'' = { fn: 'a. 'a foo -> 'b foo }
let xyz (fn:('a, 'b) fn'') =
  let fn = fn.fn in
  let res = fn a in
  let res' = fn b in
  res, res'

(* What I want: *)
type ('a, 'b) fn''' = 'a. 'a foo -> 'b foo
let xyz (fn:('a, 'b) fn''') =
  let res = fn a in
  let res' = fn b in
  res, res'


(* What I am currently using (don't pass the function around to HOFs: keep the
   computation all in a massive let statement. *)
let _ =
  let a : foo = object method bar = None method qux = None end in
  let b : bar = object method bar = None end in
  let fn a = object method bar = a#bar method qux = None end in
  (* fn gets the correctly inferred type of < bar : 'a; .. > -> < bar : 'a; qux : 'b option > *)
  fn a, fn b

关于为什么我想要的东西不可能的任何指导或理由/保证现在确实有可能/ promise 有一天——在一个更光明的世界——这会奏效，将不胜感激。

Answer 1

您想要的是在定义中多态地使用函数参数:即将 fn 应用于 xyz 中不同对象类型的两个值。这称为 Rank-2 多态性 (https://wiki.haskell.org/Rank-N_types)，但在 Hindley Milner 类型系统中不受支持(它只能处理 Rank-1)。 OCaml 基于 Hindley Milner，也不直接支持 Rank-N。

目前 OCaml 对 Rank-2 多态性的支持是非常间接的:通过多态记录成员模拟，没有任何类型推断。我想您的“另一个低于标准的解决方案”是最好的方法。