Add EitherT.liftAttemptK

[kubukoz]

No description provided.

[codecov-io]

Codecov Report

Merging #3374 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #3374      +/-   ##
==========================================
+ Coverage   93.32%   93.32%   +<.01%     
==========================================
  Files         378      378              
  Lines        7728     7729       +1     
  Branches      207      209       +2     
==========================================
+ Hits         7212     7213       +1     
  Misses        516      516

Flag	Coverage Δ
#scala_version_212	93.37% <100%> (ø)	⬆️
#scala_version_213	93.1% <100%> (ø)	⬆️

Impacted Files	Coverage Δ
core/src/main/scala/cats/data/EitherT.scala	98.5% <100%> (ø)	⬆️

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[travisbrown]

We've been removing the value-level kind-projector syntax for polymorphic functions, since there's no real possibility of it ever cross-building on Dotty, and since writing this out explicitly as:

new (F ~> EitherT[F, E, *]) { def apply[A](fa: F[A]): EitherT[F, E, A] = EitherT(F.attempt(fa)) }

…is only marginally less readable. I don't think that needs to be changed in this PR, though, since we're not actually cross-building on Dotty yet.

(Note that the type-level λ is fine on Dotty with -Ykind-projector.)

[kubukoz]

oh, I was under the impression this was also part of the -Ykind-projector feature. But it makes sense, I'll keep it in mind.

[travisbrown]

@kubukoz No, I hate kind-projector's value-level lambda syntax and couldn't be bothered to implement it in -Ykind-projector. 😄 (Also it would have required much more disruptive changes than anything else that made it into -Ykind-projector.)