自定义类型类的类型问题

Question

我在学习 Haskell 时遇到类型问题。我有一个名为 Stack 的类型类，它显然应该表现得像一个堆栈。我有一个类型 Elem，它可以是 Int 或 Bool。

现在我希望我的函数采用 Elem 的 Stack 并对其进行一些操作。结果实际上并不重要，但是我什至无法获取堆栈的第一个元素，而且我已经好几个小时都没有弄清楚出了什么问题。

import Control.Monad

data Elem = Number Int | Truth Bool deriving Eq

instance Show Elem where
  show (Number i) = show i
  show (Truth b) = show b

class Stack stack where
  push :: a -> stack a -> stack a
  top :: MonadPlus m => stack a -> m (a,stack a)
  empty :: stack a
  isEmpty :: stack a -> Bool

instance Stack [] where
  push a stack = a:stack
  top stack = if isEmpty stack then mzero else return (head stack, tail stack)
  empty = []
  isEmpty stack = if null stack then True else False

step :: Stack stack => String -> stack Elem -> Maybe (stack Elem)
step ex st = let e1 = top st :: Maybe (Elem, stack Elem)
                 --a1 = fmap fst e1
                 --e2 = top (fmap snd e1)
                 --a2 = fmap fst e2
             in Nothing

我得到的错误是

Playground.hs:22:27:
    Could not deduce (stack ~ stack2)
    from the context (Stack stack)
      bound by the type signature for
                 step :: Stack stack => String -> stack Elem -> Maybe (stack Ele
m)
      at Playground.hs:21:9-65
      `stack' is a rigid type variable bound by
              the type signature for
                step :: Stack stack => String -> stack Elem -> Maybe (stack Elem
)
              at Playground.hs:21:9
      `stack2' is a rigid type variable bound by
               an expression type signature: Maybe (Elem, stack2 Elem)
               at Playground.hs:22:23
    Expected type: stack2 Elem
      Actual type: stack Elem
    Relevant bindings include
      st :: stack Elem
        (bound at Playground.hs:22:9)
      step :: String -> stack Elem -> Maybe (stack Elem)
        (bound at Playground.hs:22:1)
    In the first argument of `top', namely `st'
    In the expression: top st :: Maybe (Elem, stack Elem)

我真的不明白为什么 Haskell 拒绝这个（甚至拒绝在我的函数中简单地调用 top st 而没有我尝试指定类型）。

我希望有人能对此有所启发！

Answer 1

正如两位评论者所建议的，您需要 -XScopedTypeVariables。

以下代码为我编译：

{-# LANGUAGE ScopedTypeVariables #-}

module Foo where

import Control.Monad

data Elem = Number Int | Truth Bool deriving Eq

instance Show Elem where
  show (Number i) = show i
  show (Truth b) = show b

class Stack stack where
  push :: a -> stack a -> stack a
  top :: MonadPlus m => stack a -> m (a,stack a)
  empty :: stack a
  isEmpty :: stack a -> Bool

instance Stack [] where
  push a stack = a:stack
  top stack = if isEmpty stack then mzero else return (head stack, tail stack)
  empty = []
  isEmpty stack = if null stack then True else False

step :: forall stack . Stack stack => String -> stack Elem -> Maybe (stack Elem)
step ex st = let e1 = top st :: Maybe (Elem, stack Elem)
                 --a1 = fmap fst e1
                 --e2 = top (fmap snd e1)
                 --a2 = fmap fst e2
             in Nothing

我不太清楚你说的"not having access to the forall syntax"是什么意思，但是如果你真的想避免它，你也可以这样写step：

step :: Stack stack => String -> stack Elem -> Maybe (stack Elem)
step ex (st :: stack Elem) = let e1 = top st :: Maybe (Elem, stack Elem)
                 --a1 = fmap fst e1
                 --e2 = top (fmap snd e1)
                 --a2 = fmap fst e2
             in Nothing

不过你仍然需要 ScopedTypeVariables。

自定义类型类的类型问题

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