[Haskell-cafe] confusion about 'instance'....

[Nicholls, Mark]

Thanks for your response, I think you helped me on one of my previous
abberations.

Hmmm....this all slightly does my head in....on one hand we have
types....then type classes (which appear to be a relation defined on
types)....then existential types...which now appear not to be treated
quite in the same way as 'normal' types....and in this instance the
syntax even seems to change....does 

"instance Num a => A a"

Mean the same thing as

"instance A (forall a.Num a=>a)"

seems like a weird questions, as you're saying my version doesn't mean
anything....but

does it mean that "forall types 'a', if 'a' is a member of the class
Num, then 'a' is a member of class 'A'"....

and secondly in what way can this construct lead to "undecidable
instances"

What are the instances, and what about them is undecidable....seems
pretty decidable to me?

What is the ramifications of turning this option on?


-----Original Message-----
From: Luke Palmer [mailto:lrpalmer at gmail.com] 
Sent: 10 January 2008 13:14
To: Nicholls, Mark
Cc: haskell-cafe at haskell.org
Subject: Re: [Haskell-cafe] confusion about 'instance'....

On Jan 10, 2008 1:03 PM, Nicholls, Mark <Nicholls.Mark at mtvne.com> wrote:
> Should be straight forward....simplest example is...
>
> class A a
>
> data D = D1
>
> instance A D
>
> fine.....D is declared to be a member of type class A....
>
> what about.....
>
> class A a
>
> type T = (forall x.Num x=>x)
>
> instance A T
>
> error!...
>
> " Illegal polymorphic or qualified type: forall x. (Num x) => x
>     In the instance declaration for `A T'"
>
> I am simply trying to state that all members of typeclass Num are of
> typeclass A....

Ahh, you want:

  instance Num a => A a

Sorry to lead you on, but that actually is not legal (and
-fallow-undecidable-instances
will make it legal, but you don't want that, because instances of this
particular form
are very likely to lead to an infinite loop).

Adding supertypes like this is not possible in Haskell.  I really want
it to be, but alas...

Luke

> Doesn't like it.
>
> Does this mean that instance only operates on 'atomic' (for want of a
> better word) types?
>
>
> -----Original Message-----
> From: Peter Verswyvelen [mailto:peter.vers at telenet.be] On Behalf Of
> Peter Verswyvelen
> Sent: 03 January 2008 12:02
> To: Nicholls, Mark
> Cc: haskell-cafe at haskell.org
> Subject: RE: [Haskell-cafe] Is there anyone out there who can
translate
> C# generics into Haskell?
>
> Hi Mark,
>
> >> "foo1 :: Int -> obj -> String"
> > Yep...I think that's what I'd do....though I would have done...
> > "foo1 :: obj -> Int -> String"
> > Does that matter?
>
> Well, it's a good habit in Haskell to move the "most important"
> parameter to
> the end of the argument list. See e.g.
> http://www.haskell.org/haskellwiki/Parameter_order.
>
> > OK but I was going to go onto
> > Interface IX<A> where A : IX<A> {}
> > and
> > Interface IX<A,B> where A : B {}
>
> No, I would not know how to that in Haskell using type classes. It
seems
> Haskell does not allow cycles in type class definitions. But as I'm
new,
> this does not mean it's not possible. It's more important to know
*what*
> you
> are trying to do, than to give a solution in a different language,
since
> OO
> and FP are kind of orthogonal languages.
>
> > Where I cannot see a way to do the above in Haskell at all....as
> > interfaces effectively operator on type classes not types....which
> seems
> > inherently more powerful
>
> Yeah, kind of makes sense. I liked interfaces in C# a lot, but when I
> started doing everything with interfaces, I found the lack of support
> for
> "mixins" or "default implementations" problematic. This ended up in a
> lot of
> copy/paste or encapsulating the implementations into a static class
with
> plain functions, a mess.
>
> > But if these could be done in Haskell the see what could be made of
> > stuff like....which is obviously problematic in C# it obviously
> doesn't
> > work....but potentially does make 'sense'.
> > Interface IX<A> : A {}
>
> Ah! That's one of the bigger restrictions in C# yes! C++ can do that;
> ATL
> uses it a lot, and I also like that approach. You can emulate "mixins"
> with
> that, and still stay in the single inheritance paradigm. In Haskell
you
> don't do that at all of course, since you avoid thinking about
"objects
> and
> inheritance" in the first place.
>
> OO is strange. They offer you the nice concept of inheritance, and
then
> the
> guidelines tell you: "don't use too many levels of inheritance"...
> Although
> I've build huge projects using OO, it always felt a bit like unsafe
> hacking.
> I don't really have that feeling with Haskell, but that could also be
> because I'm too new to the language ;-)
>
> > I'm looking at Haskell because of the formality of it's type
> > system....but I'm actually not convinced it is as powerful as an OO
> > one....i.e. OO ones operatate principally (in Haskell speak) on
"type
> > classes" not "types"
>
> Maybe you are right, I don't know, my theoritical skills are not high
> enough
> to answer that. Haskell just "feels" better to me, although the lack
of
> a
> friendly productive IDE and large standard framework remains a bit of
a
> burden.
>
> Good luck,
> Peter
>
> -----Original Message-----
> From: haskell-cafe-bounces at haskell.org
> [mailto:haskell-cafe-bounces at haskell.org] On Behalf Of Nicholls, Mark
> Sent: Wednesday, January 02, 2008 5:41 PM
> To: haskell-cafe at haskell.org
> Subject: [Haskell-cafe] Is there anyone out there who can translate C#
> generics into Haskell?
>
> I'm trying to translate some standard C# constucts into Haskell...
some
> of this seems easy....
>
> Specifically
>
> 1)
>
> Interface IX
> {
> }
>
> 2)
>
> Interface IX<A>
> {
> }
>
> 3)
>
> Interface IX<A>
>         Where A : IY
> {
> }
>
> 4)
>
> Interface IX<A> : IZ
>         Where A : IY
> {
> }
>
>
> I can take a punt at the first 2....but then it all falls apart
> _______________________________________________
> Haskell-Cafe mailing list
> Haskell-Cafe at haskell.org
> http://www.haskell.org/mailman/listinfo/haskell-cafe
> _______________________________________________
> Haskell-Cafe mailing list
> Haskell-Cafe at haskell.org
> http://www.haskell.org/mailman/listinfo/haskell-cafe
>