Hi, I wrote the regex-base API you are looking at. Uwe Schmidt wrote:
Hi all,
what's the simplest way to check, whether a given string is a wellformed regular expression?
import Text.Regex.Posix.String(compile) or import Text.Regex.Posix.ByteString(compile) etc..
In the API there's just a "mkRegex" which does not make any checks, and the "matchRegex" which throws an exception when the regex isn't wellformed.
So do I need the IO monad for checking a regex?
no. You can use the 'compile' functions above with unsafePerformIO safely. (see more detail below...)
Uwe
The error reporting in the RegexMaker type class is poor in the version you are using. Sorry about that. There are two very workable solutions. Use the actual function and avoid the type class or upgrade regex-base (and regex-posix, etc) to the newer API. The first to avoid the type class and use the particular function that it an interface to. These worker functions are all called "compile" and have sane error handling. The regex-posix backend has a 'compile' that works on String in Text.Regex.Posix.String [1] The regex-posix backend has a 'compile' that works on ByteString in Text.Regex.Posix.ByteString [2] Other backends and type are under the same organization scheme. To illustrate the usage, take the Text.Regex.Posix.String function: compile :: CompOption -> ExecOption -> String -> IO (Either WrapError Regex) This technically calls into the C library and is tagged as IO. The return type gives the most informative error I could concoct form the posix backend. This compile function is hooked into the RegexMaker type class as:
instance RegexMaker Regex CompOption ExecOption String where makeRegexOpts c e pattern = unsafePerformIO $ (compile c e pattern >>= unwrap) makeRegexOptsM c e pattern = either (fail.show) return $ unsafePerformIO $ (compile c e pattern)
And I do not think the IO is needed for safety, so unsafePerformIO is okay for this case. (But this depends on the intelligence of the c library to be thread safe. This is true on at least Mac OS X.). The second solution is to update regex-base to the darcs version at http://darcs.haskell.org/packages/regex-unstable/regex-base/ which has a newer RegexMaker API that adds what you are looking for:
-- | make using the defaultCompOpt and defaultExecOpt, reporting errors with fail makeRegexM :: (Monad m) => source -> m regex -- | Specify your own options, reporting errors with fail makeRegexOptsM :: (Monad m) => compOpt -> execOpt -> source -> m regex
These function can be used with Monad Maybe to give Nothing if there is a problem compiling the regular expression, regardless of back-end and source type. If you upgrade regex-base you will also need a newer backend, such as the updated regex-posix from under http://darcs.haskell.org/packages/regex-unstable/ Cheers, Chris [1] http://www.haskell.org/ghc/docs/latest/html/libraries/regex-posix-0.72.0.1/T... [2] http://www.haskell.org/ghc/docs/latest/html/libraries/regex-posix-0.72.0.1/T...