A method StateT.fromState turning State[A, F[B]] into StateT[F,A, B] is added.

[akopich]

No description provided.

[akopich]

@yilinwei , thank you for the discussion and encouragement.

[codecov-commenter]

Codecov Report

Merging #3524 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #3524   +/-   ##
=======================================
  Coverage   91.33%   91.33%           
=======================================
  Files         386      386           
  Lines        8588     8591    +3     
  Branches      252      260    +8     
=======================================
+ Hits         7844     7847    +3     
  Misses        744      744           

[LukaJCB]

Thanks, this seems useful :)

[travisbrown]

This is a minor thing, but we've been moving away from using Cats syntax in the core Cats implementation, in part because some cases had been causing issues on Dotty, but also because avoiding it makes runtime and compile-time performance a little more predictable. In this case I think the desugared version doesn't lose much readability:

def fromState[F[_], A, B](s: State[A, F[B]])(implicit F: Applicative[F]): StateT[F, A, B] =
  s.transformF { eval =>

    val (a, fb) = eval.value
    F.map(fb)((a, _))
  }

This definitely shouldn't block the PR, though, and I'd be happy to see it merged as-is.

[akopich]

Thanks for pointing that out. I'll desugar that. At least, for the sake of consistency with the rest of the file.

[travisbrown]

Looks good to me! I'll merge when green since the changes are minor and I don't think they should invalidate the earlier review.