无法与'Nat'匹配'Nat'

问题描述：

我试图创建一个类型来保证字符串的长度小于N个字符。无法与'Nat'匹配'Nat'

{-# LANGUAGE KindSignatures #-} 
{-# LANGUAGE OverloadedStrings #-} 
{-# LANGUAGE ScopedTypeVariables #-} 
{-# LANGUAGE DataKinds #-} 

import GHC.TypeLits (Symbol, Nat, KnownNat, natVal, KnownSymbol, symbolVal) 
import Data.Text (Text) 
import qualified Data.Text as Text 
import Data.Proxy (Proxy(..)) 

data TextMax (n :: Nat) = TextMax Text 
    deriving (Show) 

textMax :: KnownNat n => Text -> Maybe (TextMax n) 
textMax t 
    | Text.length t <= (fromIntegral $ natVal (Proxy :: Proxy n)) = Just (TextMax t) 
    | otherwise = Nothing

这给了错误：

src/Simple/Reporting/Metro2/TextMax.hs:18:50: error: 
• Couldn't match kind ‘*’ with ‘Nat’ 
    When matching the kind of ‘Proxy’ 
• In the first argument of ‘natVal’, namely ‘(Proxy :: Proxy n)’ 
    In the second argument of ‘($)’, namely ‘natVal (Proxy :: Proxy n)’ 
    In the second argument of ‘(<=)’, namely 
    ‘(fromIntegral $ natVal (Proxy :: Proxy n))’

我不明白这个错误。为什么它不起作用？ I've used almost this exact same code to get the natVal of n in other places and it works。

有没有更好的方法来做到这一点？

方备注：通过打开' PolyKinds'的扩展，ghc比poly-kinded'Proxy'更加宽松，并且错误信息变得不同，并且使得它更加清晰，它是一个典型的'ScopedTypeVariables'问题。 – gallais

答

您在textMax签名需要一个明确的forall，使ScopedTypeVariables踢和在Proxy n注释的n变得相同n在KnownNat n约束：

textMax :: forall n. KnownNat n => Text -> Maybe (TextMax n) 
textMax t 
    | Text.length t <= (fromIntegral $ natVal (Proxy :: Proxy n)) = Just (TextMax t) 
    | otherwise = Nothing

无法与'Nat'匹配'Nat'

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