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在Haskell中的ApplicativeDopragma和Applicative Functor

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Functor, Applicative, and Monad的一个例子略有变化：

{-# LANGUAGE ApplicativeDo #-}

import Safe (readMay)
-- import Control.Applicative ((<$>), (<*>))

displayAge maybeAge =
    case maybeAge of
        Nothing -> putStrLn "You provided invalid input"
        Just age -> putStrLn $ "In that year, you will be: " ++ show age

yearDiff futureYear birthYear = futureYear - birthYear

maybeAge fS bS = do 
   fI <- readMay fS
   bI <- readMay bS
   pure $ yearDiff fI bI       

main = do
    putStrLn "Please enter your birth year"
    birthYearString <- getLine
    putStrLn "Please enter some year in the future"
    futureYearString <- getLine
    displayAge $ maybeAge birthYearString futureYearString

其中 maybeAge 与 do 我用而不是

maybeAge fS bS = yearDiff <$> readMay fS <*> readMay bS

我有两个问题：

  • 在这种情况下，如何检查 maybeAge 是否使用Applicative Functor语义或Monad 1？

  • 如果使用Applicative Functor，那么在这种情况下有什么优势？

关于：ApplicativeDo .

haskell monads functor applicative

1 回答

  • 2

    我用你自己做了一个独立的例子：

    {-# LANGUAGE ApplicativeDo #-}

import Text.Read (readMaybe)

displayAge :: Maybe Int -> IO ()
displayAge maybeAge =
    case maybeAge of
        Nothing -> putStrLn "You provided invalid input"
        Just age -> putStrLn $ "In that year, you will be: " ++ show age

yearDiff :: Int -> Int -> Int
yearDiff  = (-)

maybeAge :: String -> String -> Maybe Int
maybeAge fS bS = do 
   fI <- readMaybe fS
   bI <- readMaybe bS
   pure $ yearDiff fI bI

main :: IO ()
main = do
    putStrLn "Please enter your birth year"
    birthYearString <- getLine
    putStrLn "Please enter some year in the future"
    futureYearString <- getLine
    displayAge $ maybeAge futureYearString birthYearString

    另外，在最后一行中，我交换了参数，因为它们在您的示例中看起来是错误的顺序 . 我还根据@Red的评论改进了 yearDif 的定义 .

    以下是您的问题的答案 .

    • 您可以按照GHC的User Guide中的建议，即使用 -ddump-ds 编译器开关，检查是否确实应用了applicative（和functor）操作 . 我在下面添加了几个开关，使输出更简洁 . 我还只展示有关 maybeAge 功能的摘录 .
    $ ghc appdo.hs -ddump-ds -dsuppress-type-applications -dsuppress-module-prefixes 
[1 of 1] Compiling Main             ( appdo.hs, appdo.o )

==================== Desugar (after optimization) ====================
Result size of Desugar (after optimization)
  = {terms: 75, types: 75, coercions: 0, joins: 0/0}
...
-- RHS size: {terms: 17, types: 13, coercions: 0, joins: 0/0}
maybeAge :: String -> String -> Maybe Int
[LclId]
maybeAge
  = \ (fS_a1h3 :: String) (bS_a1h4 :: String) ->
      <*>
        $fApplicativeMaybe
        (fmap
           $fFunctorMaybe
           (\ (fI_a1h5 :: Int) (bI_a1h6 :: Int) -> yearDiff fI_a1h5 bI_a1h6)
           (readMaybe $fReadInt fS_a1h3))
        (readMaybe $fReadInt bS_a1h4)
...
    • 当然，这里没有加速 . 适用于 Maybe 的操作具有不变的复杂性（ O(1) ） - 就像monadic一样 .

    在original paper中， ApplicativeDo 的作者给出了几个更复杂的monadic类型（ Haxl ， Data.Seq ，解析等）的例子，允许渐近更有效的应用操作 . 见本文第6节 .

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