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Today's Topics:
1. Re: Haskell wants the type, but I only know the class.
(Amy de Buitl?ir)
----------------------------------------------------------------------
Message: 1
Date: Mon, 26 Mar 2012 14:52:32 +0000 (UTC)
From: Amy de Buitl?ir <a...@nualeargais.ie>
Subject: Re: [Haskell-beginners] Haskell wants the type, but I only
know the class.
To: beginners@haskell.org
Message-ID: <loom.20120326t164833-...@post.gmane.org>
Content-Type: text/plain; charset=utf-8
Amy de Buitl?ir <amy <at> nualeargais.ie> writes:
> I'm trying to read something from a file, do something with it, and then write
> it out again. I don't know, or care about, the type of the object I'm reading,
> but I do know its class.
Apologies for resurrecting an old thread, but I wanted to share the solution I
came up with, in case anyone else has a similar scenario. It's not as graceful
as I'd like, but it minimises the amount of code that users of my library have
to write.
-----8<-----
{-# LANGUAGE ExistentialQuantification #-}
import Data.Binary ( Binary, encode, decode, put, get, Put, Get )
import Data.ByteString.Lazy as B ( readFile, writeFile )
import Codec.Compression.GZip ( compress, decompress )
class (Show a) => Thing a where
doSomething :: a -> IO a
label :: a -> String
data ThingBox = forall a. Thing a => TB a
putThing :: ThingBox -> Put
putThing a = do
put $ label a
put $ show a
getThing :: Get ThingBox
getThing = do
s <- get
x <- get
return $ readThingByLabel s x
instance Binary ThingBox where
put = putThing
get = getThing
instance Show ThingBox where
show (TB a) = show a
-- This trick allows me to treat ThingBoxes just like Things
instance Thing ThingBox where
doSomething (TB a) = do
a' <- doSomething a
return $ TB a'
label (TB a) = label a
readThing :: FilePath -> IO ThingBox
readThing f =
return . decode . decompress =<< B.readFile f
writeThing :: FilePath -> ThingBox -> IO ()
writeThing f = B.writeFile f . compress . encode
main = do
let a = TB Thing1
_ <- doSomething a
let b = TB Thing2
writeThing "file.txt" b
b' <- readThing "file.txt"
_ <- doSomething b
return ()
-- Now my users can develop their own things
data Thing1 = Thing1 deriving (Show, Read)
instance Thing Thing1 where
doSomething a = do
putStrLn "In Thing1's doSomething"
return a
label _ = "Thing1"
data Thing2 = Thing2 deriving (Show, Read)
instance Thing Thing2 where
doSomething a = do
putStrLn "In Thing2's doSomething"
return a
label _ = "Thing2"
-- And they'll need to write this too
readThingByLabel :: String -> String -> ThingBox
readThingByLabel "Thing1" x = TB z
where z = read x :: Thing1
readThingByLabel "Thing2" x = TB z
where z = read x :: Thing2
readThingByLabel s x = error $ "Unrecognised species: " ++ s
------------------------------
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