On Mon, 12 Aug 2013 14:50:43 +0100
Claude Heiland-Allen
Hi Brian,
On 12/08/13 03:52, briand@aracnet.com wrote: ...
Couldn't match expected type
...
Gtk.on Gtk.exposeEvent glCanvas $ \ _ -> putStrLn "foo"
...
I looked up the type of Gtk.on and exposeEvent : ... on :: object -> Signal object callback -> callback -> IO (ConnectId object) ...
I think you have the arguments flipped, try:
Gtk.on glCanvas Gtk.exposeEvent $ \_ -> ...
As for explaining the types - as I understand it, you have an object and a callback, and the Signal associates the object with the callback for a specific event type. The type variables occur more than once in the whole type for safety: the right type of callback must be provided for the event type, and the object must support the event type too.
This works _ <- Gtk.on glCanvas Gtk.exposeEvent $ return True but not this: _ <- Gtk.on glCanvas Gtk.exposeEvent fooBar where fooBar = do putStrLn "foo" return True so then I thought, aha!, all I need to do is understand the type of "return True" and all will be revealed to me. Well, it's this: Control.Monad.Trans.Reader.ReaderT (GHC.Ptr.Ptr Gtk.EExpose) IO Bool just like the error message says. Still don't know what that's supposed to be. I'm having trouble tracking down Control.Monad.Trans.Reader.ReaderT