Question

I want my showStackHead function take a stack print the head and return the rests, here is my code --code

showStackHead xx
               | xx == []   = return []
               | otherwise  = do putStrLn("result:" ++ (head xx))
                              return (tail xx)

当我管理这一法典时,汇编者告诉我,在第二次返回方面存在着一小差错,因此写这一职能的正确方式是什么?

Answer 1

2. 同意返回与StrLn一样的深度:

showStackHead xs
   | xs == []   = return []
   | otherwise  = do putStrLn ("result:" ++ (head xs))
                     return (tail xs)

Answer 2

除此以外,您的“StackHead”可以通过使用配对模式进行清洁。允许你 d清警卫的比较、头部和尾部:

#! /usr/bin/env runhaskell


showStackHead []     = return []
showStackHead (x:xs) = do
   putStrLn $ "result: " ++ [x]
   return xs


main :: IO ()
main = do
   let s1 = []
   r1 <- showStackHead s1
   putStrLn $ "returned: " ++ r1

   putStrLn "---"

   let s2 = "foo"
   r2 <- showStackHead s2
   putStrLn $ "returned: " ++ r2

   putStrLn "---"

   let s3 = "q"
   r3 <- showStackHead s3
   putStrLn $ "returned: " ++ r3

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