Minor containers API changes
Ivan Lazar Miljenovic
ivan.miljenovic at gmail.com
Mon Nov 28 21:49:54 CET 2011
On 29 November 2011 03:42, Felipe Almeida Lessa <felipe.lessa at gmail.com> wrote:
> 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 =).
I (coincidentally! honest!) vote the same as Felipe for all these.
--
Ivan Lazar Miljenovic
Ivan.Miljenovic at gmail.com
IvanMiljenovic.wordpress.com
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