编写haskell函数的正确方法是什么

Question

I'm a bit confused with an exercise. The exercice asked for a function that would take the last but one element from a list and it shows this code as the correct answer.

    myButLast :: [a] -> a
    myButLast = last . init

    myButLast' x = reverse x !! 1

    myButLast'' [x,_]  = x
    myButLast'' (_:xs) = myButLast'' xs

    myButLast''' (x:(_:[])) = x
    myButLast''' (_:xs) = myButLast''' xs

    myButLast'''' = head . tail . reverse

But I just did this and it just worked as asked

    myButLast' = init[1,2,3,4]
    myButLast'' = last myButLast'

What was all that extra code in the first solution?

Answer 1

The example code shows 5 different ways of implementing myButLast . Note that myButLast and myButLast' are entirely different functions.

About your answer - your intuition is correct, but your function only deals with one input - [1,2,3,4] . Instead, you can pass any list:

myButLast' xs = init xs
myButLast'' = last myButLast'

Combine them and add a type signature:

myButLast :: [a] -> a
myButLast xs = last $ init $ xs

Which in pointfree style is equal to the first solution:

myButLast :: [a] -> a
myButLast = last . init