Real World Haskell - 第4章 Part4 How to think about loops (続き)

Real World Haskell で Haskell を継続的に勉強中。

Chapter 4. Functional programming
Chapter 4. Functional programming

How to think about loops

• for/while ループは Haskell には存在しないので別の方法で表現する必要がある

Selecting pieces of input

oddList :: [Int] -> [Int]

oddList (x:xs) | odd x     = x : oddList xs
               | otherwise = oddList xs
oddList _                  = []

• このような操作はイディオムなので filter :: (a -> Bool) -> [a] -> [a] が使える

ghci> filter odd [3,1,4,1,5,9,2,6,5]
[3,1,1,5,9,5]

Computing one answer over a collection

• this is a “natural” way to represent a loop in a pure functional language.

mySum xs = helper 0 xs
    where helper acc (x:xs) = helper (acc + x) xs
          helper acc _      = acc

adler32_try2 xs = helper (1,0) xs
    where helper (a,b) (x:xs) =
              let a' = (a + (ord x .&. 0xff)) `mod` base
                  b' = (a' + b) `mod` base
              in helper (a',b') xs
          helper (a,b) _     = (b `shiftL` 16) .|. a