[Haskell-beginners] Haskell wants the type, but I only know the class.

[aditya siram]

Sorry for the misinformation. I should've ran the code.
-deech

On Fri, Nov 4, 2011 at 11:22 AM, Brent Yorgey <byorgey at seas.upenn.edu>wrote:

> On Fri, Nov 04, 2011 at 08:52:36AM -0500, aditya siram wrote:
> > Perhaps this is what you're looking for:
> > {-# LANGUAGE ExistentialQuantification #-}
> > import Data.Binary
> > import Data.ByteString.Lazy as B ( readFile, writeFile )
> > import Codec.Compression.GZip ( compress, decompress )
> >
> > data Thing = forall a. (Binary a, Show a, Eq a) => Thing a
> >
> > instance Binary Thing where
> >     get = get
> >     put (Thing a) = put a
> >
> > instance Show Thing where
> >     show (Thing a) = show a
> >
> > readThing :: FilePath -> IO Thing
> > readThing f = return . decode . decompress =<< B.readFile
> > f
> >
> > writeThing :: FilePath -> Thing -> IO ()
> > writeThing f = B.writeFile f . compress . encode
> >
> > doSomething :: Thing -> m Thing
> > doSomething = undefined
> >
> > main = do
> >  a <- readThing "file1.txt"
> >  a' <- doSomething a
> >  writeThing "file2.txt" a'
> >
> > It compiles on my machine (GHC 7.2.1) but I haven't tested it. It
> > uses the
>
> This will not work.  The problem is that once you have a Thing you
> cannot do anything with it, because you have no information about what
> type is inside.  In other words you cannot implement 'doSomething' to
> do anything interesting at all.  I am actually surprised that
> 'readThing' type checks -- I am not sure what type it thinks the read
> thing has, or how it can guarantee that it satisfies the given
> constraints.
>
> I tried adding a Typeable constraint to Thing and using 'cast' to
> recover the type, but that doesn't really work either.  You would
> really have to do something like changing the Binary instance for
> Thing so that it also serializes/deserializes a TypeRep along with the
> value, and then does some sort of unsafe cast after reading.
>
> You may want to take a look at how xmonad handles this problem -- it
> allows arbitrary user-extensible state and layouts, which it needs to
> serialize and deserialize when restarting itself.
>
> -Brent
>
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