Haskell中的ApplicativeDo pragma和Applicative函子

略加修改的示例如下：

{-# 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

我有两个问题：

在这种情况下，如何检查

maybeAge

是否使用Applicative Functor semantic或Monad one

如果使用Applicative Functor，在这种情况下有什么优势

---

关于：。

我用你的例子做了一个完整的例子：

{-# 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

最肯定的是，这里没有加速。

的应用操作可能具有恒定的复杂性（
O（1）
）-就像一元操作一样

在中，
ApplicativeDo
的作者给出了几个更复杂的单元类型（
Haxl
、
Data.Seq
、解析等）的示例，以允许渐进地更有效的应用操作。见本文件第6节

---

关于：。还有一个旁注。您可以使用

yearDiff=subtract

{-# 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

$ 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)
...