Add regex representations

src/Rextra/Regex.hs

@@ -0,0 +1,32 @@
+module Rextra.Regex where
+
+import Data.List
+
+data XRegex t
+  = XRToken t
+  | XRConcat [XRegex t]
+  | XRUnion [XRegex t]
+  | XRStar (XRegex t)
+  | XRPlus (XRegex t)
+  | XROptional (XRegex t)
+  deriving (Show)
+
+data Regex t
+  = REpsilon
+  | RToken t
+  | RConcat (Regex t) (Regex t)
+  | RUnion (Regex t) (Regex t)
+  | RStar (Regex t)
+  deriving (Show)
+
+toRegex :: XRegex t -> Maybe (Regex t)
+toRegex (XRToken t)      = Just $ RToken t
+--toRegex (XRConcat [r])   = toRegex r -- Optimisation, not strictly necessary
+toRegex (XRConcat rs)    = foldl' RConcat REpsilon <$> traverse toRegex rs
+toRegex (XRUnion [])     = Nothing
+--toRegex (XRUnion [r])    = toRegex r -- Optimisation, not strictly necessary
+toRegex (XRUnion (r:rs)) = foldl' RUnion <$> toRegex r <*> traverse toRegex rs
+toRegex (XRStar r)       = RStar <$> toRegex r
+toRegex (XRPlus r)       = let rr = toRegex r
+                           in  RConcat <$> rr <*> (RStar <$> rr)
+toRegex (XROptional r)   = RUnion REpsilon <$> toRegex r