Question

作为我上一个问题Using makeLenses, class constraints and type synonyms together的后续跟踪，我想了解一个新类型的错误。

类型错误是由以下示例中引入类型同义词type S = (Num n) => State n引起的。

{-# LANGUAGE TemplateHaskell #-}                                                                                                                              
{-# LANGUAGE RankNTypes #-}                                                                                                                                   

module Foo where                                                                                                                                              

import Control.Lens                                                                                                                                           

data State a = State { _a :: a                                                                                                                                
                     } deriving Show                                                                                                                          

makeLenses ''State -- Requires TemplateHaskell                                                                                                                

-- | Smart constructor enforcing class constraint on record field _a.                                                                                         
mkState :: (Num a) =>  a -> State a                                                                                                                           
mkState n = State {_a = n}                                                                                                                                    

doStuff1 :: Num a => State a -> State a                                                                                                                       
doStuff1 s = s & a %~ (*2)                                                                                                                                    

test1a = doStuff1 $ mkState  5   -- results in State {_a = 10.0}                                                                                              
test1b = doStuff1 $ mkState  5.5 -- results in State {_a = 11.0}                                                                                                                                                           

type S = (Num n) => State n -- Requires the RankNTypes extensions                                                                                             

doStuff2 :: S -> S                                                                                                                                            
doStuff2 s = s & a %~ (*2)                                                                                                                                    

test2a = doStuff2 $ mkState  5   -- Results in State {_a = 10.0}                                                                                              
--test2b = doStuff2 $ mkState  5.5 -- Type error.

如果我取消注释test2b，我会收到以下错误消息。

Could not deduce (Fractional n) arising from the literal `5.5'
from the context (Num n)
  bound by a type expected by the context: Num n => State n
  at Foo.hs:32:10-32
Possible fix:
  add (Fractional n) to the context of
    a type expected by the context: Num n => State n
In the first argument of `mkState', namely `5.5'
In the second argument of `($)', namely `mkState 5.5'
In the expression: doStuff2 $ mkState 5.5

我希望能够理解为什么引入的类型同义词会导致此错误以及如何解密错误消息。

Answer 1

S -> S不等同于forall n. Num n => State n -> State n。它相当于(forall n. Num n => State n) -> (forall n. Num n => State n)。前者意味着，对于所有数字类型n，我们可以传入State n并返回State n（对于相同类型n）。后者意味着我们为所有数字类型State n传递了n的内容，并且我们为所有类型State n找回了n的内容。换句话说，参数和结果都是多态的。

这意味着您传入的参数必须具有类型Num n => State n，而不是更具体的类型，例如State Int。这适用于5，其类型为Num n => n，但不属于5.5，其类型为Fractional n => n。

类型同义词导致类型错误

1 个答案: