Haskell 解析器、Monad 和 MonadPlus

    module Parser where

import           Control.Monad (MonadPlus, mplus, mzero)
import           Tagger        (Tag, Token)


newtype Parser a = Parser ([(Token, Tag)] -> [(a, [(Token, Tag)])])

parse :: Parser a -> [(Token, Tag)] -> [(a, [(Token, Tag)])]
parse (Parser p) = p

instance Functor Parser where
    fmap f p = do
        result <- p
        return (f result)

instance Monad Parser where
    return a = Parser (\cs -> [(a,cs)])
    p >>= f = Parser (\cs -> concat [parse (f a) cs' | (a,cs') <- parse p cs])

instance MonadPlus Parser where
    p `mplus` q = Parser (\cs -> parse p cs ++ parse q cs)
    mzero = Parser (const [])

{-

这是我的解析器代码。显然，我已经以“旧方式”完成了它，并且无法真正让它以新方式工作。你能告诉我我需要解决哪些问题才能使其正常工作吗？我阅读了这篇文章 ( https://wiki.haskell.org/Functor-Applicative-Monad_Proposal ) 并尝试更改我的代码，但我认为我在这里做错了。

我得到的编译错误:

Parser.hs:56:10:
    No instance for (Applicative Parser)
      arising from the superclasses of an instance declaration
    In the instance declaration for ‘Monad Parser’

Parser.hs:60:10:
    No instance for (GHC.Base.Alternative Parser)
      arising from the superclasses of an instance declaration
    In the instance declaration for ‘MonadPlus Parser’

编辑//

现在的代码:

module Parser where

import           Control.Applicative
import           Control.Monad (mplus, mzero, liftM, ap)
import           Tagger        (Tag, Token)

-- type Token = String
-- type Tag = String

newtype Parser a = Parser ([(Token, Tag)] -> [(a, [(Token, Tag)])])

parse :: Parser a -> [(Token, Tag)] -> [(a, [(Token, Tag)])]
parse (Parser p) = p

instance Functor Parser where
    fmap = liftM

instance Applicative Parser where
    pure a = Parser (\cs -> [(a,cs)])
    (<*>) = ap

instance Monad Parser where
    p >>= f = Parser (\cs -> concat [parse (f a) cs' | (a,cs') <- parse p cs])

instance MonadPlus Parser where --64
    p `mplus` q = Parser (\cs -> parse p cs ++ parse q cs)
    mzero = Parser (const [])

instance Alternative Parser where
    (<|>) = mplus
    empty = mzero

(+++) :: Parser a -> Parser a -> Parser a
p +++ q = Parser (\cs -> case parse (p `mplus` q) cs of
                           [] -> []
                           (x:_) -> [x])

错误:

Parser.hs:64:10:
Not in scope: type constructor or class ‘MonadPlus’

最佳答案

您可以关注 the migration guide 。简单明了:将 return 的定义移动到 pure ，添加 <*> 的样板定义并从 monad 实例中删除 return :

instance Functor Parser where
    fmap = liftM

instance Applicative Parser where
    pure a = Parser (\cs -> [(a,cs)])
    (<*>) = ap

instance Monad Parser where
    p >>= f = Parser (\cs -> concat [parse (f a) cs' | (a,cs') <- parse p cs])

对于 Alternative 它是样板文件，没有别的:

instance Alternative Parser where
    (<|>) = mplus
    empty = mzero

整个工作代码:

module Parser where

import           Control.Monad
import           Tagger        (Tag, Token)
import           Control.Applicative

newtype Parser a = Parser ([(Token, Tag)] -> [(a, [(Token, Tag)])])

parse :: Parser a -> [(Token, Tag)] -> [(a, [(Token, Tag)])]
parse (Parser p) = p

instance Functor Parser where
    fmap = liftM

instance Applicative Parser where
    pure a = Parser (\cs -> [(a,cs)])
    (<*>) = ap

instance Monad Parser where
    p >>= f = Parser (\cs -> concat [parse (f a) cs' | (a,cs') <- parse p cs])

instance MonadPlus Parser where
    p `mplus` q = Parser (\cs -> parse p cs ++ parse q cs)
    mzero = Parser (const [])

instance Alternative Parser where
    (<|>) = mplus
    empty = mzero

关于Haskell 解析器、Monad 和 MonadPlus，我们在Stack Overflow上找到一个类似的问题：https://stackoverflow.com/questions/36311809/