fgrep

import System
import List

main = do args <- getArgs
          cs <- getContents
          putStr $ fgrep (head args) cs

fgrep :: String -> String -> String
fgrep pattern cs = unlines $ filter match $ lines cs
  where
    match :: String -> Bool
    match line = any prefixp $ tails line

    prefixp :: String -> Bool
    prefixp line = pattern `isPrefixOf` line

• where
  • スコープをある式だけに制限した定義を導入する
  • where節の外側の変数・仮引数を参照できる
• 「``」による中置演算子化
  • xs `isPrefixOf` ys ＝ isPrefixOf xs ys

: head xs , tail xs , filter f xs , any f xs
: List.tails xs , List.isPrefixOf xs ys