Proposal: Rename HashMap.lookupDefault to HashMap.findWithDefault

Sat Jan 5 20:50:00 UTC 2019

Reviving this thread since we didn't reach a definitive conclusion (and its
been about a year!).

It appears that the PVP issue about the deprecation has not yet been
resolved, so there still isn't any clarification on if this would require a
major version bump or not. From re-reading the thread it appears the
largest (only?) concerns are around how to properly deprecate it without
breaking anyone's code that is compiled with -Werror.

As a partial solution, I propose that we add the new
HashMap.findWithDefault function (which brings the APIs between Map and
HashMap closer together), and keep the existing function
(HashMap.lookupDefault) but update the function comment to say that it is
deprecated and point to findWithDefault. Are there any objections? We
should eventually officially mark this function as deprecated but I don't
think we should block the addition of the alias in the meantime because
there's not currently consensus on how it should happen, I'd like to move
that to a separate discussion.

Thanks!

On Mon, Jan 29, 2018 at 6:09 PM Matt Renaud <matt at m-renaud.com> wrote:

> > I'm wondering if we should ultimately get rid of that function
> altogether, from both packages
>
> I'm on the fence about this one since its such a common operation, I
> personally would be a little surprised if it wasn't part of the API. You're
> right that it's not /that/ much code to write, but I imagine everyone will
> end up writing their own findWithDefault in their codebase, I may be wrong
> though.
>
>
> On Wed, Jan 24, 2018 at 4:27 PM, David Feuer <david.feuer at gmail.com>
> wrote:
>
>> I'm wondering if we should ultimately get rid of that function
>> altogether, from both packages. From an API design standpoint, it's kind of
>> silly, because
>>
>>     findWithDefault d k m =
>>        fromMaybe d (lookup k m)
>>
>> At present, there's a slight performance penalty to doing it that way. I
>> wonder if we can squash that now that GHC has unboxed sums.
>>
>>     lookup# :: ... => k -> m a -> (# (# #) | a #)
>>     lookup k m = case lookup# k m of
>>       (# _ | #) -> Nothing
>>       (# | a #) -> Just a
>>
>> Now case-of-case will get rid of the extra Maybe.
>>
>>
>> On Jan 23, 2018 9:15 PM, "Matt Renaud" <matt at m-renaud.com> wrote:
>>
>> *What*
>>
>> *====*
>>
>> Rename unordered-container's Data.HashMap.lookupDefault
>> <https://hackage.haskell.org/package/unordered-containers-0.2.8.0/docs/Data-HashMap-Strict.html#v:lookupDefault> to
>> findWithDefault.
>>
>> findWithDefault :: (Eq k, Hashable k) => v -> k -> HashMap k v -> v
>>
>> Note: There are no functionality changes, this is purely a rename.
>>
>>
>> *Why*
>> *===*
>>
>> This aligns the Data.HashMap API with containers'
>> Data.Map.findWithDefault
>> <https://hackage.haskell.org/package/containers-0.5.11.0/docs/Data-Map-Strict.html#v:findWithDefault>
>> .
>>
>> Data.Map.findWithDefault :: Ord k => a -> k -> Map k a -> a
>>
>> The map APIs provided by the two packages are *almost* drop in
>> replacement compatible if you aren't using any of the implementation
>> specific functions (like ordering based functions from Data.Map), this
>> change brings us one step closer.
>>
>> API consistency reduces the cognitive overhead when learning a new
>> package. Having to learn different function names for the same
>> functionality depending on which "map" implementation you're using is a
>> poor developer experience.
>>
>> We chose the containers' name findWithDefault over unordered-containers'
>> lookupDefault for two reasons:
>>
>>    1. Existing lookupX functions returns a Maybe value, while findX
>>    functions return a non-wrapped value.
>>    2. The containers package ships with GHC and is a "core" package.
>>
>>
>> *Pros:*
>> *-----*
>>
>> - Consistent API between different "map" implementations (Data.Map,
>> Data.IntMap, Data.HashMap). This makes switching implementations an import
>> change.
>> - Naming matches other similar functions (lookupX return Maybe-wrapped
>> values)
>>
>> *Cons:*
>> *-----*
>>
>> - API change requires users to update their code
>>   + unordered-containers has A LOT of users: 358815 total (13325 in the
>> last 30 days)
>>
>>
>> *How*
>> *===*
>>
>>
>> https://github.com/tibbe/unordered-containers/pull/176/commits/152f8818ee13dacb370e49b904edc4c1a4c8f87b
>>
>> *Code Changes:*
>> *-------------*
>>
>> - Rename the function in Data.HashMap.Base (and expose it from Strict and
>> Lazy modules)
>> - Make lookupDefault an INLINE alias of findWithDefault
>> - Add DEPRECATION notice to lookupDefault
>> - Bump unordered-containers version to 0.2.9.0
>>
>>
>> *Migration - Option 1:*
>>
>> *---------------------*
>>
>> *- *Announce on Haskell communication channels (haskell-cafe@,
>> haskell-community@, #haskell on Twitter, Reddit thread, etc.)
>> - Users of unordered-containers >= 0.2.9.0 receive warning about
>> deprecated function
>> - Code can be updated by find and replace:
>> s/lookupDefault/findWithDefault/
>> - lookupDefault with deprecation notice remains for 1 year (subject to
>> change)
>> - after 1 year the lookupDefault function is removed,
>> unordered-containers version bumped to 0.3.0.0 (major version bump due to
>> breaking change)
>>
>> *Migration - Option 2:*
>>
>> *---------------------*
>>
>> - Announce on Haskell communication channels (haskell-cafe@,
>> haskell-community@, #haskell on Twitter, Reddit thread, etc.)
>> - Users of unordered-containers >= 0.2.9.0 receive warning about
>> deprecated function
>> - Code can be updated by find and replace:
>> s/lookupDefault/findWithDefault/
>> - lookupDefault function is never removed
>>
>>
>> *Discussion: *
>> *===========*
>>
>> I would like to get some comments on this proposal. In particular:
>> - Is the small API churn worth the increase in consistency?
>> - Should migration option 1 (completely remove the old function) or 2
>> (keep old function indefinitely) be taken? We can punt on this and go with
>> option 2 to start and revisit later if desired.
>>
>> I hope a decision about the proposal can be reached by 2018-02-09. Thanks!
>>
>>
>>
>>
>>
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>>
>>
>>
>
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