文字列を空白で区切る

テキストファイルから入力を受け付ける時にはよく使う。あとはカンマ区切りなんかも。

import Data.Char

splitBy :: (a -> Bool) -> [a] -> [[a]]
splitBy p [] = []
splitBy p xs = a : (splitBy p $ dropWhile p $ b)
  where
    (a, b) = break p xs

splitBySpace :: String -> [String]
splitBySpace = splitBy isSpace

*Main> splitBySpace "a b c"
["a","b","c"]
*Main> splitBySpace " a b c "
["","a","b","c"]

正規表現（Text.Regex）を使えばもっとシンプルに書ける。

import Text.Regex

splitBySpace' :: String -> [String]
splitBySpace' = splitRegex (mkRegex " +")

*Main> splitBySpace' "a b c"
["a","b","c"]
*Main> splitBySpace' " a b c "
["","a","b","c",""]

……けど，ちょっと動作が違うな。
ちなみに Ruby の String#split は上の splitBySpace と同じ動作。

D:\>irb --simple-prompt
>> "a b c".split(/ +/)
=> ["a", "b", "c"]
>> " a b c ".split(/ +/)
=> ["", "a", "b", "c"]