On 2 Jun 2005 S. Alexander Jacobson
Any reason the libaries don't define:
class HasNull a where null::a->Bool class HasEmpty a where empty::a
I find that I sometimes switch between using lists, sets, or tables as my collection type and the forced import qualifification for generic collection operations seems annoying.
Probably, it would be natural for the Map and Set GHC libraries to introduce some version of class SetLike where ... My programs also switch with similar operations between sets and various tables. To unify the interface, I defined the following: ------------------------------------------------------------------- class Cardinality a where card :: a -> Natural -- (Integer) instance Cardinality (Set.Set a) where card = genericLength . Set.elems instance Cardinality (Map.Map k a) where card = genericLength . Map.keys class Cardinality a => SetLike a where sEmpty :: a sIsEmpty :: a -> Bool sUnion :: [a] -> a sMinus :: a -> a -> a sIntersect :: [a] -> a sConcat :: [a] -> a sNub :: a -> a instance Ord a => SetLike (Set.Set a) where sEmpty = Set.empty sIsEmpty = Set.null sUnion = Set.unions sMinus = Set.difference sNub = id sConcat = sUnion sIntersect [] = error "sIntersect []\n" sIntersect (s:ss) = foldl Set.intersection s ss sIncluded :: SetLike a => a -> a -> Bool sIncluded s = sIsEmpty . sMinus s instance SetLike IdList where ... ------------------------------------------------------------------- This was as under ghc-6.2.2, and now it is edited to meet 6.4. About what Set and Map libraries could provide ---------------------------------------------- The candidates may be `any', `all' analogues for Set, Map (anyway, they are simply expressed by composing with toList). direct product of sets (can be simply expressed by composing with toList, fromList). composition of maps, inverse map. On standard ----------- It is very desirable to put the Map and Set libraries to future standard Haskell library. With this, our struggle with their interface changes will end. I wonder how the Haskell language and library are going on, whether Haskell-2 is coming. Regards, ----------------- Serge Mechveliani mechvel@botik.ru