head-tail-init-last.hs

module HeadTailInitLast where
    -- middle [1] == []
    -- middle [1..6] == [2, 3, 4, 5]
    middle :: [a] -> [a]
    middle s@(_:_:_:_) = tail $ init s
    middle _ = []

    -- Maybe more Rubyish:
    middle' :: [a] -> [a]
    middle' s 
        | length s > 3 = tail $ init s 
        | otherwise = []

    -- secondToLast [1..6] == 5
    -- If list is too short, return 0:
    -- secondToLast [] == 0
    secondToLast :: [Int] -> Int
    secondToLast s@(_:_:_) = last $ init s
    secondToLast _ = 0

    -- Again:
    secondToLast' :: [Int] -> Int
    secondToLast' s
        | length s > 1 = last $ init s
        | otherwise = 0

    -- ends [1..6] == [1, 6]
    ends :: [a] -> [a]
    ends [] = []
    ends xs = [head xs, last xs]

    firstTwo :: [a] -> [a]
    firstTwo [] = []
    -- Note that `(_:_)` is the same as `[x]` because `(x:[])` == `[x]`
    -- so we need `(_:_:_)` to ensure a 2-element list0
    firstTwo s@(_:_:_) = [head s, head $ tail s]
    firstTwo s = s