[Haskell-cafe] Avoiding name collisions by using value spaces instead of modules

[Brian Hulley]

Cale Gibbard wrote:
>> Unifying these two under a single operation is certainly trickier,
>> and it's a little more questionable that it should be done at all,
>> given that their types are so different -- below is the closest I
>> could come to it off-hand.
>>
>> ---
>> {-# OPTIONS_GHC -fglasgow-exts #-} -- for fundeps/multiparameter
>> classes import qualified Data.Map as Map
>> import Data.Map (Map)
>> import qualified Data.Set as Set
>> import Data.Set (Set)
>>
>> class Insert t c a | c a -> t where
>>     insert :: t -> c a -> c a
>>
>> instance (Ord a) => Insert a Set a where
>>     insert x s = Set.insert x s
>>
>> instance (Ord k) => Insert (k,a) (Map k) a where
>>     insert (k,v) m = Map.insert k v m
>>
>> exampleSet = insert 5 $ insert 6 $ Set.empty
>> exampleMap = insert (1,2) $ insert (2,7) $ Map.empty
>>
>> ----
>>
>> Perhaps someone else will have some ideas as to suitable typeclass
>> magic to allow for the curried form rather than using tuples.
>>
>>  - Cale
>>
>
> Oh, this is a little less general, but simpler to use:
>
> {-# OPTIONS_GHC -fglasgow-exts #-}
> import qualified Data.Map as Map
> import Data.Map (Map)
> import qualified Data.Set as Set
> import Data.Set (Set)
>
> class Insert t c | c -> t where
>     insert :: t -> c -> c
>
> instance (Ord a) => Insert a (Set a) where
>     insert x s = Set.insert x s
>
> instance (Ord k) => Insert (k,a) (Map k a) where
>     insert (k,v) m = Map.insert k v m
>
> exampleSet = insert 5 $ insert 6 $ Set.empty
> exampleMap = insert (1,2) $ insert (2,7) $ Map.empty

Thanks! I'm impressed. Obviously there is a lot more power in type classes 
than I'd thought. I hadn't realised that you could separate the Ord a and 
Ord k from the type signature in the class declaration, and put them in 
instance declarations like that (for example).
It would be really interesting to see how far one could go in factoring all 
the collection type functions/values into type classes.

Best regards,
Brian.