[英]Is there a better way to have optional function arguments in Haskell?
我習慣於能夠像這樣在 Python 中定義可選參數:
def product(a, b=2):
return a * b
Haskell 沒有默認參數,但我可以通過使用 Maybe 得到類似的東西:
product a (Just b) = a * b
product a Nothing = a * 2
但是,如果您有多個參數,這會很快變得麻煩。 例如,如果我想做這樣的事情怎么辦:
def multiProduct (a, b=10, c=20, d=30):
return a * b * c * d
我必須有八個 multiProduct 定義來解釋所有情況。
相反,我決定這樣做:
multiProduct req1 opt1 opt2 opt3 = req1 * opt1' * opt2' * opt3'
where opt1' = if isJust opt1 then (fromJust opt1) else 10
where opt2' = if isJust opt2 then (fromJust opt2) else 20
where opt3' = if isJust opt3 then (fromJust opt3) else 30
這對我來說看起來很不優雅。 在 Haskell 中是否有一種更清潔的慣用方法?
也許一些漂亮的符號在眼睛上會更容易:
(//) :: Maybe a -> a -> a
Just x // _ = x
Nothing // y = y
-- basically fromMaybe, just want to be transparent
multiProduct req1 opt1 opt2 opt3 = req1 * (opt1 // 10) * (opt2 // 20) * (opt3 // 30)
如果您需要多次使用參數,我建議使用@pat 的方法。
6年后編輯
使用ViewPatterns
您可以將默認值放在左側。
{-# LANGUAGE ViewPatterns #-}
import Data.Maybe (fromMaybe)
def :: a -> Maybe a -> a
def = fromMaybe
multiProduct :: Int -> Maybe Int -> Maybe Int -> Maybe Int -> Int
multiProduct req1 (def 10 -> opt1) (def 20 -> opt2) (def 30 -> opt3)
= req1 * opt1 * opt2 * opt3
這是在 Haskell 中執行可選參數的另一種方法:
{-# LANGUAGE MultiParamTypeClasses, FlexibleInstances, FlexibleContexts #-}
module Optional where
class Optional1 a b r where
opt1 :: (a -> b) -> a -> r
instance Optional1 a b b where
opt1 = id
instance Optional1 a b (a -> b) where
opt1 = const
class Optional2 a b c r where
opt2 :: (a -> b -> c) -> a -> b -> r
instance Optional2 a b c c where
opt2 = id
instance (Optional1 b c r) => Optional2 a b c (a -> r) where
opt2 f _ b = \a -> opt1 (f a) b
{- Optional3, Optional4, etc defined similarly -}
然后
{-# LANGUAGE FlexibleContexts #-}
module Main where
import Optional
foo :: (Optional2 Int Char String r) => r
foo = opt2 replicate 3 'f'
_5 :: Int
_5 = 5
main = do
putStrLn $ foo -- prints "fff"
putStrLn $ foo _5 -- prints "fffff"
putStrLn $ foo _5 'y' -- prints "yyyyy"
更新:哎呀,我被接受了。 老實說,我認為luqui 的答案是這里最好的答案:
opt2 replicate 3 'f'
以了解我的意思)我不知道解決潛在問題的更好方法,但是您的示例可以更簡潔地編寫為:
multiProduct req1 opt1 opt2 opt3 = req1 * opt1' * opt2' * opt3'
where opt1' = fromMaybe 10 opt1
opt2' = fromMaybe 20 opt2
opt3' = fromMaybe 30 opt3
當參數變得過於復雜時,一種解決方案是為參數創建一個數據類型。 然后,您可以為該類型創建一個默認構造函數,並僅填寫您要在函數調用中替換的內容。
例子:
$ runhaskell dog.hs
Snoopy (Beagle): Ruff!
Snoopy (Beagle): Ruff!
Wishbone (Terrier): Ruff!
Wishbone (Terrier): Ruff!
Wishbone (Terrier): Ruff!
狗.hs:
#!/usr/bin/env runhaskell
import Control.Monad (replicateM_)
data Dog = Dog {
name :: String,
breed :: String,
barks :: Int
}
defaultDog :: Dog
defaultDog = Dog {
name = "Dog",
breed = "Beagle",
barks = 2
}
bark :: Dog -> IO ()
bark dog = replicateM_ (barks dog) $ putStrLn $ (name dog) ++ " (" ++ (breed dog) ++ "): Ruff!"
main :: IO ()
main = do
bark $ defaultDog {
name = "Snoopy",
barks = 2
}
bark $ defaultDog {
name = "Wishbone",
breed = "Terrier",
barks = 3
}
mcandre 和 Ionuț 提到的記錄方法的一個可能的改進/修改是使用鏡頭:
{-# LANGUAGE -XTemplateHaskell #-}
data Dog = Dog {
_name :: String,
_breed :: String,
_barks :: Int
}
makeLenses ''Dog
defaultDog :: Dog
defaultDog = Dog {
_name = "Dog",
_breed = "Beagle",
_barks = 2
}
bark :: (Dog -> Dog) -> IO ()
bark modDog = do
let dog = modDog defaultDog
replicateM_ (barks dog) $ putStrLn $
(name dog) ++ " (" ++ (breed dog) ++ "): Ruff!"
main :: IO ()
main = do
bark $ (name .~ "Snoopy") . (barks .~ 2)
bark $ (name .~ "Wishbone") . (breed .~ "Terrier") . (barks .~ 3)
或者替代地
bark :: Dog -> IO ()
bark dog = do
replicateM_ (barks dog) $ putStrLn $
(name dog) ++ " (" ++ (breed dog) ++ "): Ruff!"
main :: IO ()
main = do
bark $ name .~ "Snoopy" $ barks .~ 2 $ defaultDog
bark $ name .~ "Wishbone" $ breed .~ "Terrier" $ barks .~ 3 $ defaultDog
有關(.~)
的含義,請參見此處。
這是一種使隱式參數看起來像可選參數的方法:
{-# LANGUAGE Rank2Types, ImplicitParams #-}
multiProduct :: (Num x) => x -> ((?b::x) => x) -> ((?c::x) => x) -> ((?d::x) => x) -> x
multiProduct a b c d = let ?b=10 ; ?c=20 ; ?d=30
in a * b * c * d
test1 = multiProduct 1 ?b ?c ?d -- 6000
test2 = multiProduct 2 3 4 5 -- 120
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