Am Samstag 31 Oktober 2009 17:33:10 schrieb Joe Fredette:
You'll probably need to look at associated types/functional dependencies. The former is the new hotness, the latter is the old and not-so-busted. A quick search of the wiki ought to reveal much more than I can possibly explain, there is an example on the page for Assoc. Types about generic Map implementation, which is similar to what you're trying to do.
Or perhaps he should look at the class IArray from Data.Array.IArray, maybe he can just declare instances of IArray for his datatypes. Without more information, I can't tell which way to go.
On Oct 31, 2009, at 12:27 PM, Shawn Willden wrote:
I have a program that makes use of various data types built on top of Arrays. In some cases, they're data types that contain an Array plus some additonal information, in others, they're just "newtype" Arrays, so that I can use typechecking to make sure that I'm not using the wrong kind of object.
I'd really like to define an "ArrayOps" class with all of the operations I need, and define instances for all of the specific types. I also use some "raw" Array objects, so it would be even better if I could make an instance of my class for Array. And, ideally, I'd like to use the Array operations for my class operations.
So, I want something like:
class ArrayOps a where (!) :: a -> i -> e (//) :: a -> (i,e) -> a bounds :: a -> (i,i) range :: a -> [i]
'i' and 'e' are the index and element types, respectively.
Obviously, the signatures above reference type variables that aren't declared, and really must be constrained to be the 'i' and 'e' that were used in building the type 'a' (which is an Array i e). Something like the following (though this obviously doesn't work):
class ((Array.Array i e) a) => ArrayOps a where ...
I'm sure there must be a way to do this, but I can't figure out what the syntax would look like.
Thanks,
Shawn.