On Fri, Jun 8, 2012 at 1:33 PM, Rico Moorman wrote:

...

Or you could just use matchM :

...

       go text = case RE.matchM regex' text of           Just (before, match, after) ->               before ++ replace' match ++ go after           _ -> text

and have match be a string, just like before (since you don't use all the power of MatchText anyway).

Thank you very much for this suggestion. It works correctly. This is the final function (for reference):

regexReplace :: String -> (String -> String) -> String -> String regexReplace pattern replace text = go text    where        regex = RE.makeRegexOpts compOpts RE.defaultExecOpt pattern        compOpts = RE.compMultiline + RE.compDotAll        go text = case RE.matchM regex text of            Just (before, match, after) ->                before ++ replace match ++ go after            _ -> text

It is really interesting that the result of a function can behave like this though. I mean that a different structure is returned based on the type (inferred/given) within the dependant function, e.g. [String] or Bool ...

This is something that's possible with typeclass in Haskell and one of the way they differ from interfaces in the OO world. The classic example is the Read typeclass and "read :: (Read a) => String -> a".

One other thing I saw is that there can be a runtime error using the option compUTF8 even if the compNoUTF8Check is set too. I get:

user error (Text.Regex.PCRE.String died: (ReturnCode (-10),"Error in Text.Regex.PCRE.Wrap: ReturnCode (-10)"))

which is the error code for bad utf8 (PCRE_ERROR_BADUTF8) according to http://www.pcre.org/pcre.txt . Luckily I don't seem to need those options (yet). Would there be some way to "catch"/trace/fix those error somehow?

Fix I don't know since that's an internal error of the PCRE package or library but you can catch exceptions though only in the IO monad. In this particular case there may be an alternative since what fails is the compilation of the regex itself and makeRegexOpts has an alternative which handle errors : makeRegexOptsM . In this case writing your whole replace function in the Maybe monad would probably be a good idea :

regexMaybeReplace :: String -> (String -> String) -> String -> Maybe String regexMaybeReplace pattern replace text = do let compOpts = RE.compMultiline + RE.compDotAll regex <- RE.makeRegexOpts compOpts RE.defaultExecOpt pattern (before, match, after) <- RE.matchM regex text return $ before ++ replace match ++ regexReplace after

regexReplace :: String -> (String -> String) -> String -> String regexReplace pattern replace text = fromMaybe text (regexMaybeReplace pattern replace text)

Or something like that. -- Jedaï