Question

考虑以下类型签名：

data Foo x = Foo {
    name :: String
  , reader :: String -> x
}

instance Functor Foo where
  fmap f (Foo n r) = Foo n $ f . r

现在我展示了从Foo到optparse-applicative的{{1}}类型的自然转换：

Parser

（好吧，它有点无用，但它可以用于讨论）。

现在我将import qualified Options.Applicative as CL mkParser :: Foo a -> CL.Parser a mkParser (Foo n _) = CL.option CL.disabled ( CL.long n )作为Bar上的免费替代函数：

Foo

鉴于这是一个免费的仿函数，我应该能够将type Bar a = Alt Foo a提升为从mkParser到Bar的自然转换：

Parser

事实上，这有效并给了我一个foo :: String -> (String -> x) -> Bar x foo n r = liftAlt $ Foo n r myFoo :: Bar [String] myFoo = many $ foo "Hello" (\_ -> "Hello") clFoo :: CL.Parser [String] clFoo = runAlt mkParser $ myFoo。然而，它是一个相当无用的，因为尝试用它做很多事就会导致无限循环。例如，如果我试着描述它：

Parser

挂起直到被打断。

原因似乎是CL.cmdDesc clFoo > Chunk {unChunk = cheats在optparse-applicative和many的定义中：它使用了monadic解析。

我在这里做错了吗？我没有看到，鉴于此，可以用这种方式构造解析器。有什么想法吗？

Answer 1

正如评论中所指出的，您必须明确处理many。从Earley复制的方法：

#!/usr/bin/env stack
-- stack --resolver=lts-5.3 runghc --package optparse-applicative
{-# LANGUAGE RankNTypes #-}
{-# LANGUAGE GADTs #-}
{-# LANGUAGE ScopedTypeVariables #-}

import Control.Applicative
import qualified Options.Applicative as CL
import qualified Options.Applicative.Help.Core as CL

data Alt f a where
  Pure   :: a                             -> Alt f a
  Ap     :: f a       -> Alt f (a -> b)   -> Alt f b
  Alt    :: [Alt f a] -> Alt f (a -> b)   -> Alt f b
  Many   :: Alt f a   -> Alt f ([a] -> b) -> Alt f b

instance Functor (Alt f) where
  fmap f (Pure x)   = Pure $ f x
  fmap f (Ap x g)   = Ap x $ fmap (f .) g
  fmap f (Alt x g)  = Alt x $ fmap (f .) g
  fmap f (Many x g) = Many x $ fmap (f .) g

instance Applicative (Alt f) where
  pure = Pure

  Pure f   <*> y = fmap f y
  Ap x f   <*> y = Ap x $ flip <$> f <*> y
  Alt xs f <*> y = Alt xs $ flip <$> f <*> y
  Many x f <*> y = Many x $ flip <$> f <*> y

instance Alternative (Alt f) where
  empty = Alt [] (pure id)
  a <|> b = Alt [a, b] (pure id)
  many x  = Many x (pure id)

-- | Given a natural transformation from @f@ to @g@, this gives a canonical monoidal natural transformation from @'Alt' f@ to @g@.
runAlt :: forall f g a. Alternative g => (forall x. f x -> g x) -> Alt f a -> g a
runAlt u = go where
    go :: forall b. Alt f b -> g b
    go (Pure x)    = pure x
    go (Ap x f)    = flip id <$> u x                           <*> go f
    go (Alt xs f)  = flip id <$> foldr (<|>) empty (map go xs) <*> go f
    go (Many x f)  = flip id <$> many (go x)                   <*> go f

-- | A version of 'lift' that can be used with just a 'Functor' for @f@.
liftAlt :: (Functor f) => f a -> Alt f a
liftAlt x = Ap x (Pure id)

mkParser :: Foo a -> CL.Parser a
mkParser (Foo n r) = CL.option (CL.eitherReader $ Right . r) ( CL.long n CL.<> CL.help n )

data Foo x = Foo {
    name :: String
  , reader :: String -> x
}

instance Functor Foo where
  fmap f (Foo n r) = Foo n $ f . r

type Bar a = Alt Foo a

foo :: String -> (String -> x) -> Bar x
foo n r = liftAlt $ Foo n r

myFoo :: Bar [String]
myFoo = many $ foo "Hello" (\_ -> "Hello")

clFoo :: CL.Parser [String]
clFoo = runAlt mkParser $ myFoo

main :: IO ()
main = do
  print $ CL.cmdDesc clFoo
  print $ CL.cmdDesc $ mkParser (Foo "Hello" $ \_ -> "Hello")

从免费的替代仿函数生成optparse-applicative解析器

1 个答案: