[Haskell-cafe] object oriented technique

[Tad Doxsee]

Greg,

Thanks for your help.  Is there any significant difference between
using existential quantification

data ShapeD = forall s. ShapeC => ShapeD s

versus a GADT

data ShapeD  where
 ShapeD :: ShapeC s => s -> ShapeD

I'm not sure I understood what you meant by "You don't need to write
more typeclass instances this way."

Thanks for pointing out the Control.Exception library. It was very
helpful.  Earlier, I was trying to figure out
how to use Data.Dynamic for down-casting and couldn't get what I
wanted. The Data.Typeable usage in Control.Exception is what I was
looking for.

Tad


On Tue, Mar 29, 2011 at 12:57 AM, Gregory Collins
<greg at gregorycollins.net> wrote:
> On Tue, Mar 29, 2011 at 7:49 AM, Tad Doxsee <tad.doxsee at gmail.com> wrote:
>> class ShapeC s where
>>  draw :: s -> String
>>  copyTo :: s -> Double -> Double -> s
>>
>> -- needs {-# LANGUAGE GADTs #-}
>> data ShapeD  where
>>  ShapeD :: ShapeC s => s -> ShapeD
>>
>> Is the above the standard method in Haskell for creating an extensible
>> heterogeneous list of "objects" that share a common interface?  Are there better
>> approaches?  (I ran into a possible limitation to this approach that I plan
>> to ask about later if I can't figure it out myself.)
>
> The usual way to do this is:
>
>    {-# LANGUAGE ExistentialQuantification #-}
>    data SomeShape = forall s . ShapeClass s => SomeShape s
>
> You don't need to write more typeclass instances this way. If you give
> "SomeShape" a "ShapeClass" instance also, you can treat them
> uniformly. The downside to these approaches is that any additional
> information about the original concrete type is obliterated -- to get
> OO-style downcasting you need "Typeable" support, and it isn't free.
>
> For an example of code which uses this idiom, see the exceptions
> support from the base library:
> http://www.haskell.org/ghc/docs/latest/html/libraries/base/Control-Exception.html
>
> G
> --
> Gregory Collins <greg at gregorycollins.net>
>