プログラミングHaskell第６章練習問題　むむむむむ

prac_ch6.hs

--1
power :: Int -> Int -> Int
power m n | n < 0 = 0
          | n == 0 = 1
          | otherwise = m * power m (n-1)
{-
 power 2 3
 2 * ( power 2 2 )
 2 * 2 * ( power 2 1)
 2 * 2 * 2 * ( power 2 0)
 2 * 2 * 2 * 1
 8
-}

--2
{-　簡約しなイカ？
length [1,2,3]
1 + ( length [2,3] )
1 + 1 + ( length [3] )
1 + 1 + 1 + (length [])
1 + 1 + 1 + 0
3

drop 3 [1,2,3,4,5]
drop 2 [2,3,4,5]
drop 1 [3,4,5]
drop 0 [4,5]
[4,5]

init [1,2,3]
1 : ( init [2,3] )
1 : 2 : ( init [3] )
1 : 2 : []
[1,2]
-}

--3
myAnd :: [Bool] -> Bool
myAnd b | b == [True] = True
        | b == [False] = False
myAnd (b:bs) | b == True = and bs
             | b == False = False

myConcat :: [[a]] -> [a]
myConcat [[]] = []
myConcat ([]:xss) = myConcat xss
myConcat ((x:xs):xss) = x : myConcat (xs : xss)

myReplicate :: Int -> a -> [a]
myReplicate 0 x = []
myReplicate n x = x : myReplicate (n-1) x

doubleExclamation :: [a] -> Int -> a
doubleExclamation (x:[]) n = x
doubleExclamation (x:xs) 0 = x
doubleExclamation (x:xs) n = doubleExclamation xs (n-1)

myElem :: Eq a => a -> [a] -> Bool
myElem x [] = False

--4
merge :: Ord a => [a] -> [a] -> [a]
merge xs [] = xs
merge [] ys = ys
merge (x:xs) (y:ys) = if x<=y then  x : (merge xs (y:ys)) else y : (merge (x:xs) ys)

--5
msort :: Ord a => [a] -> [a]
msort (x:[]) = (x:[])
msort xs = merge (msort (fst pair)) (msort(snd pair))
           where pair = halve xs

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