let 在与 -XRankNTypes 一起使用时不起作用

Question

考虑以下最小示例：

{-# LANGUAGE RankNTypes #-}

module Test where

class C w

data A = A (forall u. C u => u)

x :: forall u. C u => u
x = undefined

a = A x

正如预期的那样，此类型检查正常。但是，如果 a 被重构为使用 let 语句：

{-# LANGUAGE RankNTypes #-}

module Test where

class C w

data A = A (forall u. C u => u)

x :: forall u. C u => u
x = undefined

a = let x' = x in A x'

它突然无法进行类型检查，并出现以下错误：

test.hs:12:14: error:
    * No instance for (C u0) arising from a use of `x'
    * In the expression: x
      In an equation for x': x' = x
      In the expression: let x' = x in A x'
   |
12 | a = let x' = x in A x'
   |              ^

test.hs:12:21: error:
    * Couldn't match expected type `u' with actual type `u0'
        because type variable `u' would escape its scope
      This (rigid, skolem) type variable is bound by
        a type expected by the context:
          forall u. C u => u
        at test.hs:12:19-22
    * In the first argument of `A', namely x'
      In the expression: A x'
      In the expression: let x' = x in A x'
    * Relevant bindings include x' :: u0 (bound at test.hs:12:9)
   |
12 | a = let x' = x in A x'

为什么会这样？这不违反等式推理吗？

Answer 1

这是可怕的单态限制的结果。启用 XNoMonomorphismRestriction 应该可以编译。

a = let x' = x in A x' 不等价于a = A x，因为在单态限制下x' in let x' = ...是单态的，但是A需要多态参数。

let 在与 -XRankNTypes 一起使用时不起作用

let doesn't work when used with -XRankNTypes

haskell

let

rank-n-types