[Haskell-cafe] object oriented technique
Tako Schotanus
tako at codejive.org
Tue Mar 29 11:12:31 CEST 2011
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 = ???
>
> Untill I read about existential types here:
> http://www.haskell.org/haskellwiki/Existential_type
>
> 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
>> http://www.haskell.org/haskellwiki/Heterogenous_collections
>>
>> 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.)
>>
>> - Tad
>>
>> _______________________________________________
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>> Haskell-Cafe at haskell.org
>> http://www.haskell.org/mailman/listinfo/haskell-cafe
>>
>
>
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