Minor containers API changes

[Felipe Almeida Lessa]

On Mon, Nov 28, 2011 at 2:28 PM, Milan Straka <fox at ucw.cz> wrote:
> 1) `{Map,Set}.deleteMin empty` return `empty`
>   `{IntMap,IntSet}.deleteMin empty` trigger `error "Cannot delete in empty..."`
>
>   Solutions: (a) make `{Map,Set}.deleteMin empty` throw error
>              (b) make `{IntMap,IntSet}.deleteMin empty` return empty
>
>   I vote for (b), because (a) could cause unexpected runtime errors.
>   Additionally, I expect very little programs depend on
>   `{IntMap,IntSet}.deleteMin empty` causing runtime error.

+1 for (b) as well.

> 2) `Map.deleteFind{Min,Max}` has type `Map k a -> ((k,a),Map k a)`
>   `IntMap.deleteFind{Min,Max}` has type `IntMap a -> (a, IntMap a)`
>
>   Solutions: (a) make the Map variant return only values
>              (b) make the IntMap variant return both key and value
>
>   I vote for (b), because it generalizes the original functionality.

+1 for (b) as well.

> 3) `Map.update{Min,Max}` is given a function of type `(a -> Maybe a)`
>   `Map.update{Min,Max}WithKey` is given a function of type `(key -> a -> Maybe a)`
>   `IntMap.update{Min,Max}` is given a function of type `(a -> a)`
>   `IntMap.update{Min,Max}WithKey` is given a function of type `(key -> a -> a)`
>
>   Solutions: (a) the Map variants would get a function of type `[key -> ] a -> a`
>              (b) the IntMap variants would get a function of type `[key -> ] a -> Maybe a`
>
>   I vote for (b), because it generalizes the original functionality.

+1 for (b) as well.

> 4) The functions
>   `mapKeys :: Ord k2 => (k1->k2) -> Map k1 a -> Map k2 a`
>   `mapKeysWith :: Ord k2 => (a -> a -> a) -> (k1->k2) -> Map k1 a -> Map k2 a`
>   `mapKeysMonotonic :: (k1->k2) -> Map k1 a -> Map k2 a`
>   have no IntMap correspondents.  Both `mapKeys` and `mapKeysWith`
>   can be defined by the user without loss of performance.
>
>   Solutions: (a) deprecate the `mapKeys*` methods from Map
>              (b) add the `mapKeys*` methods to IntMap.
>
>   I vote for (a). These methods are all trivial compositions and all
>   but all mapKeysMonotonic are defined as such. For mapKeysMonotonic,
>   a trivial composition with the same asymptotic complexity exists.
>   Also, if these were added to IntMap, none of them would have better
>   performance then user-defined methods.

-1 for (a).  I'd rather write 'M.mapKeys f m' than 'M.fromList $ map
(\(k,x) -> (f k, x)) $ M.toList m'.

+1 for (b).

> 5) `toDescList` exists in Map, but not in IntMap, Set or IntSet.
>
>   Solutions: (a) deprecate `Map.toDescList`
>              (b) add `toDescList` to IntMap. In this case, we should
>                  consider adding it also to Set and IntSet.
>
>   I have no strong opinion here. The `toDescList` can be trivially
>   expressed as left fold. But it is currently a subject to list fusion.
>   To vote for (a).

-1 for (a).
+1 for (b).

> Several other changes follow:
>
> 6) Result of discussion around http://hackage.haskell.org/trac/ghc/ticket/5242
>   Add
>     `Map.fromSet :: (key -> a) -> Set key -> Map key a`
>     `IntMap.fromSet :: (Int -> a) -> IntSet -> IntMap a`
>   The implementation would exploit same structure of map and set
>   (leave the shape of the original tree/trie, just adding values).
>
>   Cons: fromSet is a trivial composition:
>           fromSet f = Map.fromDistinctAscList . map (\k -> (k, f k)) . Set.toAscList
>         This can be defined by the user and is asymptotically optimal.
>   Pro: performance. Also the performance of keysSet would improve, if
>        the map can use data constructors of set.
>
>   I vote for adding these methods.

+1

> 7) Improve the generality of intersectionWith.
>   Currently the Map and IntMap define
>     intersectionWith :: Ord k => (a -> b -> c) -> Map k a -> Map k b -> Map k c
>     intersectionWithKey :: Ord k => (k -> a -> b -> c) -> Map k a -> Map k b -> Map k c
>
>   But the combining function is not general enough. Consider two
>   IntMaps storing hashable elements as (hash(element), element).
>   When intersecting elements with the same hash, the intersection can
>   be empty.
>
>   I propose to change the type of these methods to
>     intersectionWith :: Ord k => (a -> b -> Maybe c) -> Map k a -> Map k b -> Map k c
>     intersectionWithKey :: Ord k => (k -> a -> b -> Maybe c) -> Map k a -> Map k b -> Map k c
>   (and appropriately for IntMap).
>
>   Note that the combining function of differenceWith already has type `(a -> b -> Maybe a)`.

I have no strong opinions on this =).

Thanks!

-- 
Felipe.