On 2004 October 08 Friday 09:57, John Goerzen wrote:
> defaultHandler :: LogHandler b => IO (a -> IORef b)
> defaultHandler = do
> h <- (streamHandler stdout WARNING)
> r <- newIORef h
> return (\x -> r)
>
> The idea is to create a new IORef to something in the LogHandler class
> and return a function that returns it. The thing returned by that call
> to streamHandler is in the LogHandler class. Specifically, its
> declaration looks like this:
>
> instance LogHandler (GenericHandler a) where
defaultHandler returns its result for _some_ LogHandler type, while the type
declaration
> defaultHandler :: LogHandler b => IO (a -> IORef b)
has an implicit universal interpretation of b. To satisfy this type
declaration, defaultHandler would have to be able to return a handler for
_any_ LogHandler type, depending on the context in which defaultHandler is
called.
The Haskell type for defaultHandler uses an existential type, and would look
like this:
data SomeLogHandler = forall a . (LogHandler a) => SomeLogHandler a
defaultHandler :: IO (a -> IORef SomeLogHandler)
defaultHandler = do
h <- (streamHandler stdout WARNING)
r <- newIORef (SomeLogHandler h)
return (\x -> r)
Then you would use it as
f <- defaultHandler
SomeLogHandler h <- readIORef (r 0)
... use the LogHandler h ...
By the way, when you say "return a function that returns it", I suspect you
are thinking of how this would work in C or Java, where to accomplish
anything you need to call a function or invoke a method. If the function
parameter of type 'a' serves no useful purpose, then the above can be
simplified to
data SomeLogHandler = forall a . (LogHandler a) => SomeLogHandler a
defaultHandler :: IO (IORef SomeLogHandler)
defaultHandler = do
h <- (streamHandler stdout WARNING)
r <- newIORef (SomeLogHandler h)
return r
Then you would use it as
f <- defaultHandler
SomeLogHandler h <- readIORef r
... use the LogHandler h ...
_______________________________________________
Haskell-Cafe mailing list
[EMAIL PROTECTED]
http://www.haskell.org/mailman/listinfo/haskell-cafe
