escapeHTML

escapeHTML を作ってみた。

 Prelude> :set editor vi

 Prelude> :edit esc.hs

 Prelude> :load esc.hs

 [1 of 1] Compiling Main             ( esc.hs, interpreted )

 Ok, modules loaded: Main.

 Main> :!cat esc.hs

 escapeHTML :: String -> String

 escapeHTML cs = concatMap escape cs

  where

    escape :: Char -> String

    escape '&'  = "&"

    escape '\"' = """

    escape '>'  = ">"

    escape '<'  = "&lt;"

    escape c    = [c]

 

 Main> escapeHTML "aaa,\"bbb\" & <abc>"

 "aaa,&quot;bbb&quot; &amp; &lt;abc&gt;"

以下は Prelude のソース。

 http://www.haskell.org/ghc/docs/6.10.2/html/libraries/base/Prelude.html

 

 -- | Concatenate a list of lists.

 concat :: a -> [a]

 concat = foldr (++) []

 

 -- | Map a function over a list and concatenate the results.

 concatMap               :: (a -> [b]) -> [a] -> [b]

 concatMap f             =  foldr ((++) . f) []

 

 foldr            :: (a -> b -> b) -> b -> [a] -> b

 -- foldr _ z []     =  z

 -- foldr f z (x:xs) =  f x (foldr f z xs)

 {-# INLINE [0] foldr #-}

 -- Inline only in the final stage, after the foldr/cons rule has had a chance

 foldr k z xs = go xs

              where

                go []     = z

                go (y:ys) = y `k` go ys