Wed Mar 30 07:13:49 CEST 2011

```Hi Tako,

Thanks,

On Tue, Mar 29, 2011 at 2:12 AM, Tako Schotanus <tako at codejive.org> wrote:
> Sorry , the following line got lost in the copy & paste:
>    {-# LANGUAGE ExistentialQuantification #-}
>
> -Tako
>
>
> On Tue, Mar 29, 2011 at 11:09, Tako Schotanus <tako at codejive.org> wrote:
>>
>> Hi,
>> just so you know that I have almost no idea what I'm doing, I'm a complete
>> Haskell noob, but trying a bit I came up with this before getting stuck:
>>    class Drawable a where
>>       draw :: a -> String
>>    data Rectangle = Rectangle { rx, ry, rw, rh :: Double }
>>       deriving (Eq, Show)
>>    instance Drawable Rectangle where
>>       draw (Rectangle rx ry rw rh) = "Rect"
>>    data Circle = Circle { cx, cy, cr :: Double }
>>       deriving (Eq, Show)
>>    instance Drawable Circle where
>>       draw (Circle cx cy cr) = "Circle"
>>    data Shape = ???
>> And was able to complete the definition:
>>    data Shape = forall a. Drawable a => Shape a
>> Testing it with a silly example:
>>    main :: IO ()
>>    main =  do putStr (test shapes)
>>    test :: [Shape] -> String
>>    test [] = ""
>>    test ((Shape x):xs) = draw x ++ test xs
>>    shapes :: [Shape]
>>    shapes = [ Shape (Rectangle 1 1 4 4) , Shape (Circle 2 2 5) ]
>>
>> Don't know if this helps...
>> Cheers,
>> -Tako
>>
>>
>> On Tue, Mar 29, 2011 at 07:49, Tad Doxsee <tad.doxsee at gmail.com> wrote:
>>>
>>> I've been trying to learn Haskell for a while now, and recently
>>> wanted to do something that's very common in the object oriented
>>> world, subtype polymorphism with a heterogeneous collection.
>>> It took me a while, but I found a solution that meets
>>> my needs. It's a combination of solutions that I saw on the
>>> web, but I've never seen it presented in a way that combines both
>>> in a short note. (I'm sure it's out there somewhere, but it's off the
>>> beaten
>>> path that I've been struggling along.)  The related solutions
>>> are
>>>
>>> 1. section 3.6 of http://homepages.cwi.nl/~ralf/OOHaskell/paper.pdf
>>>
>>> 2. The GADT comment at the end of section 4 of
>>>
>>> I'm looking for comments on the practicality of the solution,
>>> and references to better explanations of, extensions to, or simpler
>>> alternatives for what I'm trying to achieve.
>>>
>>> Using the standard example, here's the code:
>>>
>>>
>>> data Rectangle = Rectangle { rx, ry, rw, rh :: Double }
>>>                        deriving (Eq, Show)
>>>
>>> drawRect :: Rectangle -> String
>>> drawRect r = "Rect (" ++ show (rx r) ++ ", "  ++ show (ry r) ++ ") -- "
>>>             ++ show (rw r) ++ " x " ++ show (rh r)
>>>
>>>
>>> data Circle = Circle {cx, cy, cr :: Double}
>>>                        deriving (Eq, Show)
>>>
>>> drawCirc :: Circle -> String
>>> drawCirc c = "Circ (" ++ show (cx c) ++ ", " ++ show (cy c)++ ") -- "
>>>             ++ show (cr c)
>>>
>>> r1 = Rectangle 0 0 3 2
>>> r2 = Rectangle 1 1 4 5
>>> c1 = Circle 0 0 5
>>> c2 = Circle 2 0 7
>>>
>>>
>>> rs = [r1, r2]
>>> cs = [c1, c2]
>>>
>>> rDrawing = map drawRect rs
>>> cDrawing = map drawCirc cs
>>>
>>> -- shapes = rs ++ cs
>>>
>>> Of course, the last line won't compile because the standard Haskell list
>>> may contain only homogeneous types.  What I wanted to do is create a list
>>> of
>>> circles and rectangles, put them in a list, and draw them.  It was easy
>>> for me to find on the web and in books how to do that if I controlled
>>> all of the code. What wasn't immediately obvious to me was how to do that
>>> in a library that could be extended by others.  The references noted
>>> previously suggest this solution:
>>>
>>>
>>> class ShapeC s where
>>>  draw :: s -> String
>>>  copyTo :: s -> Double -> Double -> s
>>>
>>> -- needs {-# LANGUAGE GADTs #-}
>>> data ShapeD  where
>>>  ShapeD :: ShapeC s => s -> ShapeD
>>>
>>> instance ShapeC ShapeD where
>>>  draw (ShapeD s) = draw s
>>>  copyTo (ShapeD s) x y = ShapeD (copyTo s x y)
>>>
>>> mkShape :: ShapeC s => s -> ShapeD
>>> mkShape s = ShapeD s
>>>
>>>
>>>
>>> instance ShapeC Rectangle where
>>>  draw = drawRect
>>>  copyTo (Rectangle _ _ rw rh) x y = Rectangle x y rw rh
>>>
>>> instance ShapeC Circle where
>>>  draw = drawCirc
>>>  copyTo (Circle _ _ r) x y = Circle x y r
>>>
>>>
>>> r1s = ShapeD r1
>>> r2s = ShapeD r2
>>> c1s = ShapeD c1
>>> c2s = ShapeD c2
>>>
>>> shapes1 = [r1s, r2s, c1s, c2s]
>>> drawing1 = map draw shapes1
>>>
>>> shapes2 = map mkShape rs ++ map mkShape cs
>>> drawing2 = map draw shapes2
>>>
>>> -- copy the shapes to the origin then draw them
>>> shapes3 = map (\s -> copyTo s 0 0) shapes2
>>> drawing3 = map draw shapes3
>>>
>>>
>>> Another user could create a list of shapes that included triangles by
>>> creating
>>> a ShapeC instance for his triangle and using mkShape to add it to a list
>>> of
>>> ShapeDs.
>>>
>>> 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.)
>>>