On 30-Jan-2001, Ashley Yakeley <[EMAIL PROTECTED]> wrote:
> At 2001-01-30 02:37, Fergus Henderson wrote:
>
> >class BaseClass s where
> > downcast_to_derived :: s -> Maybe Derived
>
> Exactly what I was trying to avoid, since now every base class needs to
> know about every derived class. T
On 30-Jan-2001, Marcin 'Qrczak' Kowalczyk <[EMAIL PROTECTED]> wrote:
> Tue, 30 Jan 2001 00:13:41 -0800, Ashley Yakeley <[EMAIL PROTECTED]> pisze:
>
> > How do I define downcast?
>
> You can use a non-standard module Dynamic present in ghc, hbc and Hugs
> (I don't know if it's compatible with O'H
Tue, 30 Jan 2001 00:13:41 -0800, Ashley Yakeley <[EMAIL PROTECTED]> pisze:
> How do I define downcast?
You can use a non-standard module Dynamic present in ghc, hbc and Hugs
(I don't know if it's compatible with O'Haskell).
--
__("< Marcin Kowalczyk * [EMAIL PROTECTED] http://qrczak.ids.net.
At 2001-01-30 02:37, Fergus Henderson wrote:
>class BaseClass s where
> downcast_to_derived :: s -> Maybe Derived
Exactly what I was trying to avoid, since now every base class needs to
know about every derived class. This isn't really a practical way to
build an extensible type hierarch
On 30-Jan-2001, Ashley Yakeley <[EMAIL PROTECTED]> wrote:
> At 2001-01-17 17:03, Lennart Augustsson wrote:
>
> >You seem to want dynamic type tests.
...
> >You might want to look at existential types; it is a similar feature.
>
> I seem to run into a similar problem:
>
> --
> class BaseClass s
At 2001-01-17 17:03, Lennart Augustsson wrote:
>You seem to want dynamic type tests. This is another feature, and
>sometimes a useful one. But it requires carrying around types at
>runtime.
Yes. I tried to do that myself by adding a field, but it seems it can't
be done.
>You might want to lo
Ashley Yakeley wrote:
> This seems to be stretching the concept of 'subtype'.
I don't think so, this is the essence of subtyping.
> Sorry if I sound so bitter and disappointed. I was hoping for a Haskell
> extended with real dynamic subtyping...
You seem to want dynamic type tests. This is a
At 2001-01-17 16:07, Johan Nordlander wrote:
>Ashley Yakeley wrote:
>>
>> OK, I've figured it out. In this O'Haskell statement,
>>
>> > struct Derived < Base =
>> > value :: Int
>>
>> ...Derived is not, in fact, a subtype of Base. Derived and Base are
>> disjoint types, but an implicit map
Ashley Yakeley wrote:
>
> OK, I've figured it out. In this O'Haskell statement,
>
> > struct Derived < Base =
> > value :: Int
>
> ...Derived is not, in fact, a subtype of Base. Derived and Base are
> disjoint types, but an implicit map of type "Derived -> Base" has been
> defined.
>
> --
At 2001-01-16 14:36, Tom Pledger wrote:
>Here's a similar example using type-substitution overlapping:
>
>instance TheValue Char where ...
>instance Monad m=> TheValue (m Char) where ...
>instance TheValue a => TheValue (Maybe a) where ...
>
>trouble = theV
Ashley Yakeley writes:
[...]
> Subtyping-overlapping is quite different from type-substitution
> overlapping.
Different, but with some similarities.
> Consider:
>
> struct B
>
> struct D1 < Base =
> a1 :: Int
>
> struct D2 < Base =
> a2 :: Int
>
> class TheValue a w
At 2001-01-16 14:04, Tom Pledger wrote:
>The subtyping (struct Derived < Base ...) makes the two instances
>overlap, with 'instance TheValue Derived' being strictly more specific
>than 'instance TheValue Base'. If the system preferred the less
>specific one, the more specific one would never be
Ashley Yakeley writes:
> At 2001-01-16 13:18, Magnus Carlsson wrote:
>
> >f1 = Just 3
> >f2 = f3 = f4 = Nothing
>
> So I've declared b = d, but 'theValue b' and 'theValue d' are different
> because theValue is looking at the static type of its argument?
>
> What's to stop 'instance Th
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