XorTInstances + cleanup tests

[yilinwei]

Add Semigroup and Monoid for XorT.

Also, cleanup the tests so the implicit resolution is clear.

[codecov-io]

Current coverage is 88.64%

Merging #1106 into master will increase coverage by 0.01%

1. 2 files (not in diff) in ...main/scala/cats/free were modified. more
   • Hits +7
2. 2 files in ...main/scala/cats/data were modified. more
   • Misses +1
   • Hits +1
3. File ...ats/std/option.scala (not in diff) was modified. more

@@             master      #1106   diff @@
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  Files           227        227          
  Lines          2946       2959    +13   
  Methods        2890       2904    +14   
  Messages          0          0          
  Branches         53         52     -1   
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+ Hits           2611       2623    +12   
- Misses          335        336     +1   
  Partials          0          0          

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[non]

Looks good to me! Thanks! 👍

[yilinwei]

As @peterneyens pointed out, the Applicative in this case isn't the same as dervied from the Monad (which one doesn't extend the other).

EDIT2:

Thought about this further and I think that it's not possible to have a Applicative for XorT because it's a monad transformer which means that it needs to be dependent on the other context to change the computation flow.

If XorT could have an Applicative it wouldn't make any sense because it would have to have the same one as F[Xor[?]] which is not what we want from any monad transformer.

Because of that, I'm going to remove the Applicative instance.

EDIT:

So I played around with this, and interestingly enough:

val f = XorT(List("error".left[Int => Int], "error".left[Int => Int]))
val a = XorT(List(42.right[String], "another error".left[Int]))
//f.ap(a) returns List(Left("error"), Left("error"), Left("error"), Left("error"))
//f.flatMap(f => a.map(f)) returns List(Left("error"), Left("error"))

Going to fix this tomorrow.

[xuwei-k]

Thought about this further and I think that it's not possible to have a Applicative for XorT

Yes, it's not possible. see scalaz/scalaz#255

[yilinwei]

I've removed the Applicative.

I think it may be good to add a helper method onto XorT to find the Nested applicative like in scalaz but that's a different issue.

[ceedubs]

minor: I think this could simply be that.flatMap(this.map)

[ceedubs]

👍 thanks!

@non does this still look good to you?

[non]

LGTM 👍