[Haskell-cafe] object oriented technique

[Gábor Lehel]

On Wed, Mar 30, 2011 at 6:52 AM, Tad Doxsee <tad.doxsee at gmail.com> wrote:
> 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

The difference is purely syntactical. Use whichever you like better.

(There may be portability ramifications. I'm not sure if other
compilers implement ExistentialQuantification and/or GADTs.)

>
> 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>
>>
>
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