Add ContravariantCartesian typeclass and for Tuple2 add Monad and FlatMap

[johnynek]

These new instances should be isomorphic to WriterT[Id, A, B] instances, but there may be cases where you already have a tuple2 and want to use a monad.

This approach can be generalized to TupleN, but I am not sure it yet justifies code-gen or macros for this purpose.

Interestingly, one could make a Monad for a general case-class that is generic in at least the last parameter, but that may be going too far.

I also added Cartesian[Eq] to make the tests easier (otherwise I needed to plumb some new implicits everywhere).

[codecov-io]

Codecov Report

Merging #1299 into master will decrease coverage by 0.03%.
The diff coverage is 90.24%.

@@            Coverage Diff             @@
##           master    #1299      +/-   ##
==========================================
- Coverage   90.66%   90.63%   -0.04%     
==========================================
  Files         236      237       +1     
  Lines        3684     3725      +41     
  Branches       56       58       +2     
==========================================
+ Hits         3340     3376      +36     
- Misses        344      349       +5

Impacted Files	Coverage Δ
core/src/main/scala/cats/InvariantMonoidal.scala	77.77% <0%> (-15.56%)	❌
core/src/main/scala/cats/Composed.scala	96% <100%> (+0.34%)	✅
core/src/main/scala/cats/instances/tuple.scala	100% <100%> (ø)	✅
...main/scala/cats/laws/discipline/FlatMapTests.scala	100% <100%> (ø)	✅
...re/src/main/scala/cats/functor/Contravariant.scala	100% <100%> (ø)	✅
core/src/main/scala/cats/Cartesian.scala	75% <100%> (+8.33%)	✅
laws/src/main/scala/cats/laws/FlatMapLaws.scala	100% <100%> (ø)	✅
...e/src/main/scala/cats/ContravariantCartesian.scala	83.33% <83.33%> (ø)
core/src/main/scala/cats/functor/Strong.scala	33.33% <0%> (-66.67%)	❌
core/src/main/scala/cats/MonadFilter.scala	7.14% <0%> (-42.86%)	❌
... and 12 more

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[erik-stripe]

LGTM 👍

[adelbertc]

Either! :D

[adelbertc]

#1289 is merged now - looks like this change doesn't have a merge conflict but I suspect if merged it will break the build :)

[johnynek]

I'll update, get the coverage positive and try again.
On Fri, Aug 19, 2016 at 17:18 Adelbert Chang notifications@github.com
wrote:

#1289 #1289 is merged now - looks
like this change doesn't have a merge conflict but I suspect if merged it
will break the build :)

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[johnynek]

I think this is green now, except for the PR failing because of the Xor => Either. Since that is being reverted, this should be green after #1305 goes through.

[adelbertc]

👍

[kailuowang]

👍