some-list-function.hs

length' :: String -> Int
length' = foldl (\ x _ -> x + 1) 0

last' :: Show a => [a] -> (Maybe a)
last' = foldl (\ _ x -> Just x) Nothing

head' :: Show a => [a] -> (Maybe a)
head' [] = Nothing
head' (x:_) = Just x

tail' :: Show a => [a] -> (Maybe [a])
tail' [] = Nothing
tail' (_:xs) = Just xs

substring' :: Show a => Int -> Int -> [a] -> (Maybe [a])
substring' _ _ [] = Nothing
substring' start end xs
    | start > end          = Nothing
    | (length xs) <= start = Nothing
    | otherwise            = Just $ take (end - start) $ drop start xs

main :: IO ()
main = do
        -- length' test
        print "length' test =========="
        print $ length' "hogehoge"
        print $ length' "fuga"
        print $ length' ""

        -- last' test
        print "last' test =========="
        print $ last' [1, 2, 3]
        print $ last' "abcdef"
        print $ last' ""
        print $ last' ([]::[Int])

        -- head' test
        print "head' test =========="
        print $ head' [1, 2, 3]
        print $ head' "abcdef"
        print $ head' ""
        print $ head' ([]::[Int])

        -- tail' test
        print "tail' test =========="
        print $ tail' [1, 2, 3]
        print $ tail' "abcdef"
        print $ tail' ""
        print $ tail' ([]::[Int])

        -- substring' test
        print "substring' test =========="
        print $ substring' 0 2 [1, 2, 3]
        print $ substring' 1 3 "abcdef"
        print $ substring' 1 5  [1, 2, 3]
        print $ substring' 4 5  [1, 2, 3]
        print $ substring' 2 1  [1, 2, 3]