Haskell parsec令牌类型条件

Question

考虑最小化的代码：

module Parser where

import Text.ParserCombinators.Parsec
import Text.Parsec.Pos

oneTokenP f = token show (\_ -> initialPos "Dummy") f
oneToken t = token show (\_ -> initialPos (show t)) 
                  (\t' -> if t == t' then Just () else Nothing)

我得到了错误：

Parser.hs:8:1: error:
    • Non type-variable argument
        in the constraint: Text.Parsec.Prim.Stream
                             s Data.Functor.Identity.Identity a1
      (Use FlexibleContexts to permit this)
    • When checking the inferred type
        oneTokenP :: forall u s a a1.
                     (Show a1,
                      Text.Parsec.Prim.Stream s Data.Functor.Identity.Identity a1) =>
                     (a1 -> Maybe a) -> Text.Parsec.Prim.Parsec s u a

Parser.hs:9:1: error:
    • Non type-variable argument
        in the constraint: Text.Parsec.Prim.Stream
                             s Data.Functor.Identity.Identity a
      (Use FlexibleContexts to permit this)
    • When checking the inferred type
        oneToken :: forall u s a.
                    (Eq a, Show a,
                     Text.Parsec.Prim.Stream s Data.Functor.Identity.Identity a) =>
                    a -> Text.Parsec.Prim.Parsec s u ()

我违反了哪种打字条件？

重写了以下@am合金建议：

oneTokenP f = token showTok posFromTok testTok
 where
     showTok (pos,t) = show t
     posFromTok (pos,t) = initialPos "Dummy"
     testTok (pos,t) = f t
oneToken x = token showTok posFromTok testTok
 where 
     showTok (pos,t) = show t
     posFromTok (pos,t) = initialPos (show t)
     testTok (pos,t) = if x == t then Just () else Nothing

Answer 1

将您的函数与the example in the documentation for token进行比较

 mytoken x
   = token showTok posFromTok testTok
   where
     showTok (pos,t)     = show t
     posFromTok (pos,t)  = pos
     testTok (pos,t)     = if x == t then Just t else Nothing

在每种情况下，输入都是位置和标记的元组。你正试图把这样一个元组当作只是一个象征。我不清楚为什么这个错误会产生你所得到的特定错误，但我认为你仍然可以看到这是一个错误。

Answer 2

错误信息表明

(Use FlexibleContexts to permit this)

所以，我们可以加上

{-# LANGUAGE FlexibleContexts #-}

在文件的顶部。在那之后，它会编译得很好。