2013年2月20日水曜日

開発環境

Real World Haskell』(Bryan O'SullivanJohn GoerzenDon Stewart(著)、山下 伸夫伊東 勝利株式会社タイムインターメディア(翻訳)、オライリー・ジャパン、2009年、ISBN978-4-87311-423-3)の4章(関数プログラミング)の4.6(ループをどのように考えるか)の練習問題5.を解いてみる。

5.

コード(BBEdit)

Sample.hs

-- file: Sample.hs
myTakeWhile :: (a -> Bool) -> [a] -> [a]
myTakeWhile p (x:xs) | p x = x:(myTakeWhile p xs)
                     | otherwise = []
myTakeWhile p [] = []

myTakeWhile' :: (a -> Bool) -> [a] -> [a]
myTakeWhile' p xs = foldr step [] xs
    where step x ys | p x = x:ys
                    | otherwise = []

a = [1,2,3,4,5]
b = [1,3,2,4,5]
c = [2,1,3,4,5]
d = []
e = [1]
f = [2]

入出力結果(Terminal)

$ ghci
GHCi, version 7.4.2: http://www.haskell.org/ghc/  :? for help
Loading package ghc-prim ... linking ... done.
Loading package integer-gmp ... linking ... done.
Loading package base ... linking ... done.
Prelude> :load Sample.hs
[1 of 1] Compiling Main             ( Sample.hs, interpreted )
Ok, modules loaded: Main.
*Main> takeWhile odd a
[1]
*Main> myTakeWhile odd a
[1]
*Main> myTakeWhile' odd a
[1]
*Main> takeWhile odd b
[1,3]
*Main> myTakeWhile odd b
[1,3]
*Main> myTakeWhile' odd b
[1,3]
*Main> takeWhile odd c
[]
*Main> myTakeWhile odd c
[]
*Main> myTakeWhile' odd c
[]
*Main> takeWhile odd d
[]
*Main> myTakeWhile odd d
[]
*Main> myTakeWhile' odd d
[]
*Main> takeWhile odd e
[1]
*Main> myTakeWhile odd e
[1]
*Main> myTakeWhile' odd e
[1]
*Main> takeWhile odd f
[]
*Main> myTakeWhile odd f
[]
*Main> myTakeWhile' odd f
[]
*Main> :quit
Leaving GHCi.
$

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