On Thursday 20 May 2010 15:49:59, Roly Perera wrote:

Hi,

I'm looking for a simple way to rewrite strings according to simple composable rules like:

replace "_" by "\\(\\hole\\)" replace "-n" where n matches an integer by "^{n}"

so that I can import some pretty-printed output into a LaTeX alltt environment. I'm guessing that this nice functional stream transformation problem has been solved thousands of times. Could anyone point me to a simple package that would do this for me?

many thanks, Roly

Hmm, I'm a little surprised that the Regex libraries don't seem to provide replacing functions (at least my short search didn't find any). Still, shouldn't be too hard to use one of their matching functions to write a replacing function. Or start from scratch: repFun :: (String -> Maybe (String, String)) -> (String -> String) -> String -> String repFun test repl str@(c:cs) = case test str of Nothing -> c : repFun test repl cs Just (match, rest) -> repl match ++ repFun test repl rest repFun _ _ [] = [] -- uses: repFun uscore (const "\\(\\hole\\)") where uscore ('_':rest) = Just ("_",rest) uscore _ = Nothing repFun dashDigs expo where dashDigs ('-':rest@(d:_)) | isDigit d = Just (span isDigit rest) dashDigs _ = Nothing expo digs = '^':'{':digs ++ "}"

... to rewrite strings according to simple composable rules like ...

more often than not, regexp/replace is a hack to avoid the "real thing" (parse -> AST -> transform -> AST' -> print). such hacks tend to grow into something that is definitely not composable. but, you can be a hero ... http://xkcd.com/208/