[Haskell-beginners] Escaping special characters in text

[Thomas Koster]

Stefan,

On Thu, Jun 11, 2015 at 03:53:41PM +1000, Thomas Koster wrote:
> My program needs to escape and unescape "special characters" in text
> (Data.Text.Text), using my own definition of "special character"
> (isSpecial :: Char -> Bool). I am looking for a library that provides
> functions that implement or help me implement this functionality. I
> don't really care exactly how the special characters are escaped, but
> my preference is to prefix them with backslashes.

On 11 June 2015 at 16:55, Stefan Höck <efasckenoth at gmail.com> wrote:
> The answer to your question depends on whether your program needs
> additional functionality. If the only thing you need to do is taking
> special characters and escaping them with an escape character plus a
> substitute character, this can be done with very little code using
> functions from Data.Text:
>
>   import Data.Text (Text)
>   import qualified Data.Text as T
>
>   -- Character used for escaping
>   ec :: Char
>   ec = '$'
>
>   -- Replace a character to be escaped with its substitute
>   escapeChar :: Char -> Char
>   escapeChar = id
>
>   -- Inverse of escapeChar
>   unescapeChar :: Char -> Char
>   unescapeChar = id
>
>   -- True if given char needs to be escaped
>   isSpecial :: Char -> Bool
>   isSpecial = ('?' ==)
>
>   -- Escape chars in a given text
>   escape :: Text -> Text
>   escape = T.concatMap handleChar
>     where handleChar c | isSpecial c = T.pack [ec, escapeChar c]
>                        | otherwise   = T.singleton c
>
>   -- Unescape chars in a given text
>   unescape :: Text -> Text
>   unescape t = case T.break (ec ==) t of
>                  (a,b) | T.null b  -> a
>                        | otherwise -> let b' = T.tail b
>                                           e  = unescapeChar $ T.head b'
>                                       in T.append a $
>                                          T.cons e $ unescape (T.tail b')

Thank you for your response.

Yes, this is all I need to do. I had completed about two thirds of a
similar implementation before becoming concerned that I was spending
too much time reinventing this particular wheel and that there may be
much easier and/or shorter ways to do this using a library written by
a wheel surgeon.

The only substantial difference is that my own version uses and
returns a Data.Text.Lazy.Builder so that texts can be streamed and
spliced into larger texts without copying (I am also using Chris
Done's formatting library [1]), but only if I get it right, of course,
which is another reason why I started to look around for libraries by
Haskellers more experienced than I.

> If on the other hand you want to escape special characters with blocks of text
> (instead of single characters as in my code) you probably also need a
> second character to mark the end of an escape. Even then, the code
> should not get much more involved than the example above.

I don't need more functionality re the escaping itself; your
implementation is a valid example that provides the essence of what I
need.

[1] https://hackage.haskell.org/package/formatting

--
Thomas Koster