XMonadContrib.ReadMap: a Read instance of Map for GHC 6.4 users

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+module XMonadContrib.ReadMap () where
+
+{- An instance of Read for Data.Map.Map's; useful for people that are still
+ - compiling under 6.4.  To use it, add the following line to StackSet.hs:
+ - import XMonadContrib.ReadMap
+ -}
+
+import Data.Map (Map, fromList)
+import GHC.Read
+
+instance (Ord k, Read k, Read e) => Read (Map k e) where
+  readsPrec _ = \s1 -> do
+    ("{", s2) <- lex s1
+    (xs,  s3) <- readPairs s2
+    ("}", s4) <- lex s3
+    return (fromList xs, s4)
+
+-- parses a pair of things with the syntax a:=b
+-- stolen from the GHC 6.6 sources
+readPair :: (Read a, Read b) => ReadS (a,b)
+readPair s = do (a, ct1)    <- reads s
+                (":=", ct2) <- lex ct1
+                (b, ct3)    <- reads ct2
+                return ((a,b), ct3)
+
+readPairs :: (Read a, Read b) => ReadS [(a,b)]
+readPairs s1 = case readPair s1 of
+    [(p, s2)]   -> case s2 of
+                        (',':s3)    -> do
+                            (ps, s4) <- readPairs s3
+                            return (p:ps, s4)
+                        _           -> [([p], s2)]
+    _           -> [([],s1)]