[Haskell] Read Instances for Data.Map and Data.Set

[Georg Martius]

Hi folks,

I was really annoyed by the fact that for Data.Map and Data.Set are no Read 
instances declared, but Show instances are! I believe there should be some 
kind of unwritten rule that in the standart lib the Show and Read instances 
come pairwise and are fully compatible. 

Anyway since there was no response to S. Alexander Jacobson post [1], I 
decided to write these instances at least for GHC in the style of the other 
instances in GHC.Read.
For the {a,b,...} syntax I have used a modified version of @list@ from 
GHC.Read
Is there a chance of having this in the libs?

Cheers,
 Georg

[1] http://www.haskell.org//pipermail/haskell/2005-February/015380.html

import qualified Data.Map as Map
import qualified Data.Set as Set
import Text.Read
import qualified Text.Read.Lex as L
import GHC.Read

instance (Ord k, Read k, Read e) => Read (Map.Map k e) where
    readPrec = do elements <- set (readPair readPrec readPrec)
                  return $ Map.fromList elements

-- ^ @(readPrec p1 p2)@ parses a pair of things with the syntax @a:=b@ 
--  where @a@ is parsed by @p1@,
--    and @b@ is parsed by @p2@
readPair :: ReadPrec a -> ReadPrec b -> ReadPrec (a,b)
readPair reada readb =
    do a <- reset reada
       L.Symbol ":=" <- lexP
       b <- reset readb
       return (a,b)

instance (Ord a, Read a) => Read (Set.Set a) where
    readPrec = do elements <- set (readPrec)
                  return $ Set.fromList elements

set :: ReadPrec a -> ReadPrec [a]
-- ^ @(set p)@ parses a list of things parsed by @p@,
-- using the curly-braces syntax.
set readx =
    parens 
    ( do L.Punc "{" <- lexP
         (setRest False +++ setNext)
    )
 where
  setRest started =
    do L.Punc c <- lexP
       case c of
         "}"           -> return []
         "," | started -> setNext
         _             -> pfail
  
  setNext =
    do x  <- reset readx
       xs <- setRest True
       return (x:xs)


-- 
---- Georg Martius,  Tel: (+49 34297) 89434 ----
------- http://www.flexman.homeip.net ---------
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