More flexible TransLift

[djspiewak]

Reimplements TransLift to no longer fix the type constructor. This is substantially more flexible and makes it possible to achieve the same generality as scalaz's Hoist. This is done while retaining the flexibility of cats's previous implementation, which is that liftT requires the minimal constraint necessary for the target MT. In most cases, this is Functor, but in some cases (like Kleisli) it is nothing at all (i.e. no constraint).

To handle the "nothing at all" case, I have added a Trivial type, which is basically the unit typeclass. It takes no parameters and has a single instance which is guaranteed to be in the implicit scope. Several type aliases are provided to allow Trivial to be used in slots of varying shapes. A F[_] typeclass (e.g. Monoid) can be satisfied by Trivial.P1, and a F[_[_]] typeclass (e.g. Monad) can be satisfied by Trivial.PH1. I'm very open to nicer naming.

Oh, and for students of SI-2712, the way that TLExtract is encoded (specifically with SingletonMT and SingletonM) demonstrates an interesting workaround to some of its foibles. One concern here is that the error messages will be horrendous. But without control over the type toString in @implicitNotFound, we can't really do better.

This PR addresses #917.

[djspiewak]

Er… what? The build is hanging? This change definitely increases the compilation time of TransLiftTests, but it's not increased by that much. Furthermore, the hang appears to be happening at runtime. I did confirm that TransLiftTests runs to completion locally, so that can't be it. Anyone have any idea what's going on?

[DavidGregory084]

@djspiewak yeah, this seems to be a big problem at the minute, see the discussion on #938, #810, #942 about fixing this

[djspiewak]

@DavidGregory084 Whew! Off the hook. :-)

[DavidGregory084]

Does this doc comment need to be updated to refer to Trivial?

[djspiewak]

Yes. Will fix.

[adelbertc]

In light of the fixed build (#954) this need to be re-merged against the latest master. Sorry about that!

[codecov-io]

Current coverage is 90.98%

Merging #940 into master will increase coverage by +0.01% as of 037add7

@@            master    #940   diff @@
======================================
  Files          180     181     +1
  Stmts         2150    2153     +3
  Branches        42      42       
  Methods          0       0       
======================================
+ Hit           1956    1959     +3
  Partial          0       0       
  Missed         194     194       

Review entire Coverage Diff as of 037add7

Powered by Codecov. Updated on successful CI builds.

[ceedubs]

This looks good to me so far. @djspiewak would you be willing to make that comment update to refer to Trivial and to add a test or two so that that the codecov isn't reporting a coverage drop?

[djspiewak]

@ceedubs Maybe I'm misreading the coverage report, but as far as I can tell, there's literally nothing to add. All of the testable lines are already tested. The reason the coverage is dropping is I added untestable lines (literally: whitespace and type-level declarations), which for some reason, Codecov includes in the computation which generates the coverage percentage.

Again, I may simply be misreading it. If someone can point out to me what I missed, I will happily write tests. :-)

[adelbertc]

It looks that way to me as well. 👍 from me!

[adelbertc]

Hm given the fancy nature of these types, could you also add some tests in tests/src/test/scala/cats/tests/SyntaxTests.scala ? Not for code coverage, but just to check that they can be used nicely and make sure nobody breaks anything in the future.

[djspiewak]

Well so much for your 👍! :-)

More seriously, the syntax is already tested by what previously existed. I could probably add one or two things that are now supported but which weren't before, but it wouldn't be too different from the tests we already have.

[adelbertc]

I was halfway through amending my comment when I saw your new one :-)

Yeah I just realized there are pre-existing tests, I wrongly assumed this was a completely new addition. My 👍 stands!

[ceedubs]

👍 thanks, @djspiewak!