Added minimal WriterT implementation.

[sungiant]

No description provided.

[ceedubs]

Should we change T[_] to F[_]? If memory serves, everywhere else we are pretty much using F[_] (and G[_] if there are two type constructor params)

[ceedubs]

Thanks for submitting this! WriterT is a great addition to Cats.

I left some comments, but in general this looks great to me.

We should hook this up to some law-checking and tests. If you would be willing to do that on this PR, that would be great. Otherwise, I'm happy to work on that.

One other small stylistic note: in other parts of Cats we aren't putting a space in front of ( for defs and function calls. We probably should have a Scalastyle rule enforcing consistency for that, but in the meantime, would you be willing to match the convention?

Thanks again, @sungiant!

[sungiant]

Hi @ceedubs, thanks for the feedback, I've made a few stylistic changes as per your suggestions.

I've not made the Kind-Projector change as I'm not very familiar with how to make the change, how would ({type WT[$] = WriterT[F, L, $]})#WT[A] be rewritten?

As for tests, I took a look, but I don't fully understand how the law-checking part works, it would be great if someone in the know could get the ball rolling.

[codecov-io]

Current coverage is 75.56%

Merging #542 into master will increase coverage by +0.14% as of 2d70740

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  Files          152     154     +2
  Stmts         2055    2091    +36
  Branches        68      68       
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+ Hit           1550    1580    +30
  Partial          0       0       
- Missed         505     511     +6

Review entire Coverage Diff as of 2d70740

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[ceedubs]

@sungiant thank you. You should be able to rewrite new Monad[({type WT[$] = WriterT[F, L, $]})#WT] as new Monad[WriterT[F, L, ?]] using kind-projector. I can add the law-checking (and maybe some other tests), but I probably won't be able to get around to it until mid-to-late next week.

[ceedubs]

@sungiant I've submitted a PR to your repo that uses kind projector and adds some law-checking and a couple tests.

Some things that we should probably do (but I think could wait until subsequent PRs):

• Add a Writer alias for when F is Id
• Add some more instances (Functor, Applicative, Bifunctor, Semigroup, Monoid, etc) along with law-checking for those instances
• Add more ad-hoc tests

[ceedubs]

👍

Thanks @sungiant!

[non]

Looks great, thanks!

👍