On 17-Feb-1999, S.D.Mechveliani <[EMAIL PROTECTED]> wrote:
> Who could please, explain a bit existential types?
>
> It is required to organise a table with the key
>
> data K = K1 | K2 | K3 {- ... -} deriving(Eq,Ord,Enum)
>
> to put/extract there the items of different types, say, 'a' and
> ('a','b') as well. Is this possible?
Yes. But what do you want to do with the values once you've extracted them?
> Understanding nothing in this subject, i tried
>
> data KTab = forall a. KT (FiniteMap K a)
That gives you a FiniteMap whose values are all of the same type.
Probably what you really want is something like
data Value = forall v. MkValue v
type KTab = FiniteMap K Value
This gives you single FiniteMap into which you can put values of
different types, so long as you wrap those values using `MkValue'.
You can also extract them from the finite map. But once you've
extracted them, you can't really do anything useful with them. If
you want to be able to do something useful with them, e.g. printing
them, then you need to add the appropriate typeclass constraints
to the definition of `Value':
data Value = forall v. show v => MkValue v
^^^^^^^^^
> f :: KTab -> KTab
> f (KT t) = case addToFM t K1 'a' of
> t' -> KT (addToFM t' K2 ('a','b'))
This would become
f :: KTab -> KTab
f t = case addToFM t K1 (MkValue 'a') of
t' -> addToFM t' K2 (MkValue ('a','b'))
Here I've added the `MkValue' wrappers and deleted the unnecessary `KT'
wrappers.
> And this does not compile. Then, try
>
> class Tab t k where lkp :: t a -> k -> Maybe a
> addT :: t a -> k -> a -> t a
>
> instance Tab KTab K where lkp (KT t) k = lookupFM t k
>
> Also rejected. How to handle with this?
I'm not sure what you're trying to achieve here.
But maybe you wanted something like this,
class Tab t k v where
lkp :: t -> k -> Maybe v
addT :: t -> k -> v -> t
instance Tab KTab K where
lkp = lookupFM
...
or perhaps this
class Tab t k where
lkp :: t -> k -> Maybe Value
addT :: t -> k -> Value -> t
-- instance definition same as before
?
--
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WWW: <http://www.cs.mu.oz.au/~fjh> | but source code lives forever"
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