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)
when I run this code, the compiler tells me there is a parse error about the second return, so what is the right way to write this function?
Indent the 'return' to the same depth as 'putStrLn', like so:
showStackHead xs
| xs == [] = return []
| otherwise = do putStrLn ("result:" ++ (head xs))
return (tail xs)
As an aside, your showStackHead could be cleaner by using pattern matching. Allowing you to ditch the guard comparison, head and tail:
#! /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
