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Getting rid of `Data.Monoid.DecidablyEmpty`

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#44 6 comments 0 reactions 0 assignees View on GitHub

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Assessment

Difficulty
4/5
Estimated time
3-5 days
Newbie friendliness
35/100
Issue type
Refactor
Clarity
Mostly clear
Activity status
Stale
Tech stack
haskell
Domain
backend

Research direction

Start by reading Data.Monoid.DecidablyEmpty and comparing its API with Data.Monoid.Null from monoid-subclasses. Check the dependent-map instance and then follow the listed migration steps: update the alias and deprecations, remove the old module, and release the affected packages.

Written by the indexing model from the issue text.

Description

There's a comment at the top of Data.Monoid.DecidablyEmpty which reads: -- TODO upstream somwhere else?

That "somewhere else" already exists: Data.Monoid.Null from monoid-subclasses.

I had a look at the instances it provides, and it is missing the following ones:

instance MonoidNull a => MonoidNull (Identity a)
instance MonoidNull a => MonoidNull (WrappedMonoid a)
instance (Ord a, Bounded a) => MonoidNull (Max a)
instance (Ord a, Bounded a) => MonoidNull (Min a)
instance MonoidNull (Proxy a)
instance MonoidNull a => MonoidNull (Const a b)
instance MonoidNull a => MonoidNull (Down a)
instance MonoidNull p => MonoidNull (Par 1 p)
instance MonoidNull (U1 p)
instance MonoidNull (f p) => MonoidNull (Rec1 f p)
instance MonoidNull (f p) => MonoidNull (M1 i c f p)
instance MonoidNull c => MonoidNull (K1 i c p)
instance (MonoidNull (f p), MonoidNull (g p)) => MonoidNull ((f :*: g) p)
instance (MonoidNull (f p), MonoidNull (g p)) => MonoidNull ((f :.: g) p)
instance (...) => MonoidNull (a, b, c, d, e)

-- Should go to dependent-map
instance GCompare k => MonoidNull (DMap k v)

This would solve #42 but is too much work to hold up GHC 9.2 support. Tasks, as I see them:

  • Add the missing instances to monoid-subclasses and release
  • Add the missing instance to dependent-map and release
  • Replace Data.Monoid.DecidablyEmpty.DecidablyEmpty with type alias, set isEmpty = Data.Monoid.Null.null, deprecate both, and release
  • Remove deprecated Data.Monoid.DecidablyEmpty.

None of this seems particularly controversial. We might want monoid-subclasses anyway because of #37. It's quite a light package, so depending on it from dependent-map shouldn't be a problem either.

Dominant language
Haskell
Stars
17
Forks
17
Avg merge
9h 32m
Merged PRs (30d)
1

Getting set up

First steps

  1. Read the whole issue, then the project's contributing guide.
  2. Comment on the issue to say you are picking it up — it saves two people doing the same work.
  3. Fork the repository and make your change on a branch.
  4. Open a pull request that references the issue number.

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