simulating dynamic dispatch

[Simon Peyton-Jones]

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: oleg@pobox.com [mailto:oleg@pobox.com]
| Sent: 21 March 2003 04:19
| To: hdaume@ISI.EDU; haskell@haskell.org
| Subject: Re: simulating dynamic dispatch
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| > i'm hoping to be able to simulate a sort of dynamic dispatch based
on
| > class instances.
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| It seems you want to dispatch based not on a type but on the
| constraint of a type.
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| You code almost worked. Here's the a bit updated and working version.
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| class Foo a where { foo :: a -> Bool }
| class Bar a where { bar :: a -> Bool }
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| data FB =3D forall a . Foo a =3D> MkFoo a | forall a . Bar a =3D> =
MkBar a
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| instance Foo FB where
|     foo (MkFoo x) =3D foo x
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| instance Bar FB where
|     bar (MkBar x) =3D bar x
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| -- some instances for the test
| instance Foo Int where
|     foo x =3D x =3D=3D 0
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| instance Bar Char where
|     bar x =3D x =3D=3D 'a'
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| test x =3D case x of
|           (MkFoo a) -> Just $ foo a
|           (MkBar a) -> Just $ bar a
| --	  _         -> Nothing
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| -- *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
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