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

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 existential quantification extension to constrain a datatype to the typeclasses you mention. The caveat is that the only functions you can run on "a" or "a'" are those defined by the Eq, Show and Binary typeclasses. -deech On Fri, Nov 4, 2011 at 8:26 AM, Antoine Latter wrote:

On Fri, Nov 4, 2011 at 7:51 AM, Amy de Buitléir wrote:

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Antoine Latter writes:

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A really simple and common way to do this would be using a sum-type:

Here's what I'm trying to accomplish. I want to write a daemon that will cycle through the files, load each one in turn, and invoke the doSomething method. I would like to be able to allow people to use my daemon with any assortment of new types that they create, as long as that type implements the doSomething, readThing, and writeThing methods.

I could make the sum-type approach work, but then users would have to modify the type. And there might be dozens of different types in use, some provided by me, and some provided by the user, so it would get messy. But I might have to go with that approach.

An executable, once compiled, cannot really learn about new types without dynamically loading new object code. Which is possible but often tricky.

You'd have this problem in other languages, too, except Java and .Net have spent a lot of time working on the dynamic loading of new object code into a running process (but you would still need to get the .class file or .dll into a folder where the executable can read it).

Maybe I'm mis-understanding your requirements, though.

Antoine

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