On Wed, Mar 30, 2011 at 6:52 AM, Tad Doxsee 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 wrote:

...

On Tue, Mar 29, 2011 at 7:49 AM, Tad Doxsee 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...

G -- Gregory Collins

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