Data.Map.mapKeysMonotonic is a misleading name

[Artyom Kazak]

It's not used _very_ often, but it does appear a few times on Hackage. For instance, it's used several times in Agda source: https://codesearch.aelve.com/haskell/search?query=.mapKeysMonotonic&filter=Data.Map(+|$)&insensitive=off&space=off&precise=off&sources=on&page=1

On Apr 4 2019, at 3:19 pm, Andreas Abel <andreas.abel at ifi.lmu.de> wrote:
> > I doubt this function is used very often.
>
> You could be mistaken. Me, too. Without statistical data, I have 0
> confidence in such estimates of brain 1.
> https://en.wikipedia.org/wiki/Thinking,_Fast_and_Slow
>
> On 2019-04-04 14:46, Elliot Cameron wrote:
> > Well I'm proposing deprecation not removal. The deprecation could even
> > live forever for all I care. It would point to the new name as well. Not
> > to mention I doubt this function is used very often.
> >
> > On Thu, Apr 4, 2019, 2:58 AM Andreas Abel <andreas.abel at ifi.lmu.de
> > <mailto:andreas.abel at ifi.lmu.de>> wrote:
> >
> > I am not sure this subtlety is worth the breakage and annoyance to
> > users
> > coming with the name change.
> > If you think about it, renaming the keys while preserving the tree
> > structure cannot work if two old keys map to the same new key.
> >
> > On 2019-04-03 17:13, Elliot Cameron wrote:
> > > Yeah the clearest names seem to be really long ones:
> > > mapKeysMonotonicDistinct, mapKeysInjectiveIncreasing,
> > > mapKeysReferToDocs, etc.
> > >
> > > On Wed, Apr 3, 2019 at 11:11 AM David Feuer
> > <david.feuer at gmail.com <mailto:david.feuer at gmail.com>
> > > <mailto:david.feuer at gmail.com <mailto:david.feuer at gmail.com>>> wrote:
> > >
> > > We can't use "increasing" because mathematically that usually
> > means
> > > non-strictly increasing. Using "ascending" gets us in trouble
> >
> > with
> > > the other functions in the module that use the word in a
> >
> > non-strict
> > > sense.
> > >
> > > On Wed, Apr 3, 2019, 10:46 AM Elliot Cameron
> > <eacameron at gmail.com <mailto:eacameron at gmail.com>
> > > <mailto:eacameron at gmail.com <mailto:eacameron at gmail.com>>> wrote:
> > >
> > > Hello!
> > > In some recent analysis I ran into a subtlety that caught
> > me by
> > > surprise: Data.Map.mapKeysMonotonic has a misleading name.
> > >
> > > A monotonic function is not a strictly /increasing/ function,
> > > but merely non-decreasing. However,
> >
> > |mapKeysMonotonic| requires
> > > that it's mapping function be injective, which means it
> >
> > really
> > > only supports /increasing/ functions.
> > >
> > > valid (mapKeysMonotonic (\x-> if x`elem` [1,2]then 2
> > else x) (fromList [(1,"a"), (2,"b"), (3,"c")]))== False
> > >
> > > The docs hint at this with "This means that @f
> > > <https://github.com/f>@ maps distinct original keys to
> >
> > distinct
> > > resulting keys."
> > >
> > > However, I'd propose that we deprecate this name and
> > rename to
> > > something like |mapKeysIncreasing|or |mapKeysAsc| (to
> >
> > follow the
> > > pattern of other *Asc functions). We should also clarify
> >
> > the docs.
> > >
> > >
> > > From https://github.com/haskell/containers/issues/617
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