Excellent paper! I have started using some of the techniques already!
But, back to the real question :). I now no longer think such a thing is
possible. Essentially what I want (given 'cast :: Typeable a, Typeable b
=> a -> Maybe b' [1]), is to write something like:
test :: forall a r . Typeable a =>
(forall b . (Typeable b, Foo b) => b -> r) ->
a -> Maybe r
test f a = liftM f (cast a)
but then 'b' is ambiguous. I'm not sure there is any way around
this. Can someone help?
- Hal
[1] is there any reason why parens are requires around class
contexts? i've always found this odd...
--
Hal Daume III | [EMAIL PROTECTED]
"Arrest this man, he talks in maths." | www.isi.edu/~hdaume
On Fri, 21 Mar 2003, Simon Peyton-Jones wrote:
> You might also find the 'cast' function in Section 3 of "Scrap your
> boilerplate" useful.
> http://research.microsoft.com/~simonpj/papers/hmap/
> I'm not certain, but it has the right smell.
> Simon
>
> | -----Original Message-----
> | From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]
> | Sent: 21 March 2003 04:19
> | To: [EMAIL PROTECTED]; [EMAIL PROTECTED]
> | Subject: Re: simulating dynamic dispatch
> |
> |
> | > i'm hoping to be able to simulate a sort of dynamic dispatch based
> on
> | > class instances.
> |
> | It seems you want to dispatch based not on a type but on the
> | constraint of a type.
> |
> | You code almost worked. Here's the a bit updated and working version.
> |
> | class Foo a where { foo :: a -> Bool }
> | class Bar a where { bar :: a -> Bool }
> |
> | data FB = forall a . Foo a => MkFoo a | forall a . Bar a => MkBar a
> |
> | instance Foo FB where
> | foo (MkFoo x) = foo x
> |
> | instance Bar FB where
> | bar (MkBar x) = bar x
> |
> | -- some instances for the test
> | instance Foo Int where
> | foo x = x == 0
> |
> | instance Bar Char where
> | bar x = x == 'a'
> |
> |
> | test x = case x of
> | (MkFoo a) -> Just $ foo a
> | (MkBar a) -> Just $ bar a
> | -- _ -> Nothing
> |
> |
> | -- *Main> test $ MkFoo (0::Int)
> | -- Just True
> | -- *Main> test $ MkFoo (10::Int)
> | -- Just False
> | -- *Main> test $ MkBar 'a'
> | -- Just True
> | -- *Main> test $ MkBar 'b'
> | -- Just False
> |
> | _______________________________________________
> | Haskell mailing list
> | [EMAIL PROTECTED]
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>
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