The other option is {-# LANGUAGE ExistentialQuantification #-} data Renderer s = Renderer { initialize :: IO s, destroy :: IO (), renderS :: SystemOutput -> s -> IO s } -- Now, you need to hold the state somewhere, which you can do with an existential: data InitializedRenderer = forall s. IRenderer s (Renderer s) initRenderer :: Renderer s -> IO InitializedRenderer initRenderer r = do s <- initialize r return (IRenderer s r) render :: InitializedRenderer -> SystemOutput -> IO InitializedRenderer render (IRenderer s r) o = do s' <- renderS r o s return (IRenderer s' r) On Sat, Sep 3, 2011 at 10:44 PM, M. George Hansen <technopolitica@gmail.com>wrote:

On Sat, Sep 3, 2011 at 12:33 AM, Max Rabkin <max.rabkin@gmail.com> wrote:

...

On Sat, Sep 3, 2011 at 03:15, M. George Hansen <technopolitica@gmail.com> wrote:

...

Greetings,

I'm a Python programmer who is relatively new to Haskell, so go easy on me :)

I have a program that uses (or will use) plugins to render output to the user in a generic way. I'm basing the design of the plugin infrastructure on the Plugins library, and have the following interface:

data Renderer = Renderer { initialize :: IO (), destroy :: IO (), render :: SystemOutput -> IO () }

How about having initialize return the render (and destroy, if necessary) functions:

initialize :: IO (SystemOutput -> IO ())

or

initialize :: IO (SystemOutput -> IO (), IO())

Thanks for your reply. That does seem like the best solution, I'll give it a try.

-- M. George Hansen

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