[Haskell-cafe] ATs [The container problem]

[Andrew Coppin]

Antoine Latter wrote:
> Sure!  Using type-families:
>
>   
>> class Container c where
>>    type Elem c
>>    insert :: Elem c -> c -> c
>>     
>
>   
>> instance Container [a] where
>>    type Elem [a] = a
>>    insert = (:)
>>     
>
>   
>> instance Container ByteString where
>>    type Elem ByteString = Word8
>>    insert = BS.cons
>>     
>
>   
>> instance Ord a => Container (Set a) where
>>    type Elem (Set a) = a
>>    insert = Set.insert
>>     
>
>   

That's more or less how I was hoping it works. (Was unsure of the actual 
syntax, and the documentation is rather terse.)

So there's a class called Container that has a _type_ that is 
_associated_ with it, representing the type of the elements? And you can 
set that type to be either a type variable or an explicit type?

> Now the hard part is coming up with a proper API and class hierarchy.
>   

So... exactly like in every OOP language in existence then? ;-)