Minimizing typeclass surface in cats-kernel

[denisrosset]

As per #1995

[codecov-io]

Codecov Report

Merging #1997 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1997      +/-   ##
==========================================
+ Coverage   95.24%   95.26%   +0.01%     
==========================================
  Files         301      301              
  Lines        4922     4919       -3     
  Branches      123      125       +2     
==========================================
- Hits         4688     4686       -2     
+ Misses        234      233       -1

Impacted Files	Coverage Δ
kernel/src/main/scala/cats/kernel/Hash.scala	90% <ø> (ø)	⬆️
core/src/main/scala/cats/instances/eq.scala	100% <100%> (ø)	⬆️
core/src/main/scala/cats/data/WriterT.scala	94.62% <100%> (ø)	⬆️
core/src/main/scala/cats/data/Nested.scala	96.49% <100%> (ø)	⬆️
...e/src/main/scala/cats/instances/partialOrder.scala	100% <100%> (ø)	⬆️
laws/src/main/scala/cats/laws/discipline/Eq.scala	94.23% <100%> (ø)	⬆️
...rnel/src/main/scala/cats/kernel/PartialOrder.scala	90.9% <100%> (+2.67%)	⬆️
core/src/main/scala/cats/instances/order.scala	100% <100%> (ø)	⬆️
kernel/src/main/scala/cats/kernel/Eq.scala	93.75% <100%> (-0.19%)	⬇️
kernel/src/main/scala/cats/kernel/Order.scala	83.33% <100%> (-0.46%)	⬇️
... and 1 more

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[LukaJCB]

Can you rebase with master? :)

[kailuowang]

this wil fail Mima test , need to add some exceptions here

[oscar-stripe]

Thanks for making these changes!

[kailuowang]

[info] Compiling 78 Scala sources to /home/travis/build/typelevel/cats/tests/.js/target/scala-2.11/test-classes...
[error] /home/travis/build/typelevel/cats/tests/src/test/scala/cats/tests/SortedSetSuite.scala:17: value reverse is not a member of cats.kernel.PartialOrder[scala.collection.immutable.SortedSet[Int]]
[error] checkAll("PartialOrder[SortedSet[Int]].reverse", PartialOrderTests(PartialOrder[SortedSet[Int]].reverse).partialOrder)
[error] ^
[error] /home/travis/build/typelevel/cats/tests/src/test/scala/cats/tests/SortedSetSuite.scala:18: value reverse is not a member of cats.kernel.PartialOrder[scala.collection.immutable.SortedSet[Int]]
[error] checkAll("PartialOrder[SortedSet[Int]].reverse.reverse", PartialOrderTests(PartialOrder[SortedSet[Int]].reverse.reverse).partialOrder)
[error] ^
[error] two errors found
[error] (testsJS/test:compileIncremental) Compilation failed

[denisrosset]

@kailuowang Rebased with the latest master, should solve the SortedSetSuite problem.

PS: my git-fu is minimal, how should I combine the above commits to avoid polluting the history?

[denisrosset]

We should also remove the tests corresponding to the old .reverse.reverse test, as we no longer perform the optimization that the reverse of the reverse is the original object.

Note: I could restore this optimization by pattern matching on the type of derived Order, at the price of adding new classes.

[kailuowang]

Its okay to have multiple trivial commits in a PR. We will squash merge into one commit. I myself don't see it necessary to optimize .reverse.reverse. Testing it against the law doesn't hurt though right?

[denisrosset]

It doesn't hurt, but it's unnecessary, as the .reverse.reverse test was covering an optimization that is no longer there, so we are basically testing that -(-x) = x as the Order.reverse method cannot be overloaded. Your call.