プログラミングHaskell第４章練習問題　むむむずい

prac_ch4.hs

--1
halve :: [a] -> ([a], [a])
halve xs = (take len xs, drop len xs)
  where
    len = fromIntegral (length xs) `div` 2

--2
-- if
safetail :: (Eq a) => [a] -> [a]
safetail xs = if xs == [] then [] else drop 1 xs

-- guard
safetail2 :: (Eq a) => [a] -> [a]
safetail2 xs | xs == []  = []
             | otherwise = drop 1 xs

-- pattern
safetail3 :: [a] -> [a]
safetail3 [] = []
safetail3 xs = drop 1 xs

--3
-- if
orfunc :: Bool -> Bool -> Bool
orfunc a b = if a == False && b == False then False else True

-- guard
orfunc2 :: Bool -> Bool -> Bool
orfunc2 a b | a == False && b == False = False
            | otherwise = True

-- pattern
orfunc3 :: Bool -> Bool -> Bool
orfunc3 False False = False
orfunc3 _ _ = True

--pattern2
orfunc4 :: Bool -> Bool -> Bool
orfunc4 True True = True
orfunc4 True False = True
orfunc4 False True = True
orfunc4 False False = False

--3
andfunc :: Bool -> Bool -> Bool
andfunc a b = if a == True then
                if b == True then True else False
                else False

--4
andfunc2 :: Bool -> Bool -> Bool
andfunc2 a b = if a == True then b else False

--5
-- (\x -> \y -> \z -> x * y * z)