On Thursday 26 May 2011 02:23:06, Federico Mastellone wrote:
Hi,
I created a Data.MultiMap module based on Data.Map and Data.Set like this:
data MultiMap k v = MultiMap (Map k (Set v))
and a Data.IntMultiMap module based on Data.IntMap and data.IntSet like this:
data IntMultiMap = IntMultiMap (IntMap IntSet)
Both of these would better be newtypes instead of data, I think. Using data incurs some run-time overhead (the newtype doesn't exist at run- time, only during compile-time [type checking phase], so it's a strictly controlled type alias in practice, making it easier [or possible at all] to apply optimisations available for the underlying type) due to the extra indirections via the constructor and introduces the new value (MultiMap _|_), which complicates strictness analysis and optimisations in general.
For example the functions to add a value I wrote are:
For MultiMap: addValue :: k -> v -> MultiMap k v -> MultiMap k v addValue k v (MultiMap m) = MultiMap $ Map.insertWith (\new old -> Set.insert v old) k (Set.singleton v) m
For IntMultiMap: addValue :: Int -> Int -> IntMultiMap -> IntMultiMap addValue k v (IntMultiMap m) = IntMultiMap $ IntMap.insertWith (\new old -> IntSet.insert v old) k (IntSet.singleton v) m
Both modules look almost the same, with the same documentation, same behavior, same function names but with different type signatures.
Well, we have the same situation with Map/IntMap and Set/IntSet, so
Is there a way to make this simpler?
Not a really good one (at least, none I know).
The same thing happens to the modules that are using MultiMap and IntMultiMap, I have to write two versions of each.
You can reduce the code duplication at the use sites with a type class, {-# LANGUAGE TypeFamilies #-} class MultiMapClass m where type Key m type Value m empty :: m singleton :: Key m -> Value m -> m addValue :: Key m -> Value m -> m -> m ... instance (Ord k, Ord v) => MultiMapClass (MultiMap k v) where type Key (MultiMap k v) = k type Value (MultiMap k v) = v empty = MultiMap Map.empty ... instance MultiMapClass IntMultiMap where type Key IntMultiMap = Int type Value IntMultiMap = Int empty = IntMultiMap IntMap.empty ...
Thanks!