On Sun, Aug 30, 2009 at 9:14 PM, Tom Poliquin
Chaddaï Fouché wrote:
data Volumic = Volume a => V a
I've never used existentials .. but this seems like a powerful idea. Unfortunately I couldn't get this to compile .. It was unhappy about 'data Volumic' so I changed it to 'data Volumic a' .. it was still unhappy and took me down the road of compiler switch options .. until I had .. ghc -XFlexibleInstances -XExistentialQuantification -XEmptyDataDecls --make Main.hs which was also unsuccessful.
Oops... Sorry, I just put together a simple example and didn't test it, I forgot the forall :
data Volumic = forall a . (Volume a) => V a
You must explicitly quantify the type variable to do existentials (and use the proper extensions, which you did).
Philosophical Summary ... All the examples of type classes examples I've seen in tutorials and books look simple, beautiful and elegant. No disrespect intended to the coding suggestions but they seem a little more difficult than I had expected for my toy problem ..
So I'm wondering why that is ..
- I'm stupidly trying to shoehorn my toy problem into a type class example which is not the best approach.
Well it is a _toy_ problem where you're specifically trying to shoehorn something into the form you wished to discover, this kind of thing sometimes seems harder to do than resolve real problems where the context and the practical objectives give you clues all along.
- The problem is *not* a toy problem and is really complicated.
In fact you're trying to address the so called "Expression problem" here, it isn't a simple feat and while Haskell bring some answer they are not so straightforwardly supported and easy to use as could be wished (though I'm unaware of any practical language that do better currently IMHO).
- Type classes are more appropriate at the 'system' level than the 'application' level. Applications are better off using algebraic data types.
Depend on what you call "Application" I guess, given that creating a big application in Haskell seems to consist of creating a framework/dsl to express the main program in two lines, it may be that you'll still need type class for that, but it's true that type classes are better placed in libraries than in the application code, where algebraic/record type and pattern matching are often a more appropriate solution.
- Tom (me) has expectations that are too high. I do have high expectations of Haskell. I've written several mid sized applications (obviously without using type classes :-) ) and found them easy to write and unbelievably easy to refactor!!!
It may be that too, while Haskell is impressive, it isn't perfect just yet. ;) -- Jedaï