[Haskell-cafe] Design question
lrpalmer at gmail.com
Wed Dec 16 16:15:03 EST 2009
On Wed, Dec 16, 2009 at 9:40 AM, haskell at kudling.de <haskell at kudling.de> wrote:
> i am not quite sure how to do this in the most elegant way:
> I have some data structures:
> data A = A Double
> data B = B Double
> data C = C Double
> and i want to allow only a subset in another data structure, so i did
> something like this:
> data SubSet = SubSetA A | SubSetC C
I think you might be looking for too much sugar. I don't know much
about your problem, but I would use approximately your approach and be
type SubSet = Either A C
> and use it in Foo:
> data Foo = Foo [SubSet]
> No i want to perform a polymorphic operation on the contents of A,B,C, e.g.
> doSomething :: Foo -> [Double]
> doSomething (Foo s) = map doSomethingElse s
doSomething (Foo s) = map (doSomethingWithA ||| doSomethingWithC) s
(||| is from Control.Arrow)
If that gets too complicated, you can build the "doSomething"
functions in a type-directed way using typeclasses:
class DoSomething a where
doSomething :: a -> Double
instance DoSomething A where ...
instance DoSomething B where ...
instance DoSomething C where ...
instance (DoSomething a, DoSomething b) => DoSomething (Either a b) where
doSomething = doSomething ||| doSomething
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