Minor containers API changes

Joachim Breitner mail at joachim-breitner.de
Mon Nov 28 21:24:45 CET 2011


I’ll skip on the +1 and only make negative comments, as they are
fewer :-)

Am Montag, den 28.11.2011, 17:28 +0100 schrieb Milan Straka:
> 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.

They seem useful to me to write legible code, therefore I’m in favor of
(b). Also, they are used in the wild (random reference):

> 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).

I have at least one project where I make heavy use of list fusion. In
that case I only use toList, but one could imagine a case where I want
that order. Therefore, I’m in favor of (b).

> 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.

I very much agree (guess I don’t want to be only negative after all).

Greetings and thanks for your work,

Joachim "nomeata" Breitner
  mail at joachim-breitner.de  |  nomeata at debian.org  |  GPG: 0x4743206C
  xmpp: nomeata at joachim-breitner.de | http://www.joachim-breitner.de/

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