8000 Syntax extensions for Parallel require Monad[M], but implementation doesn't · Issue #2254 · typelevel/cats · GitHub
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Syntax extensions for Parallel require Monad[M], but implementation doesn't #2254

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oleg-py opened this issue May 10, 2018 · 3 comments · Fixed by #3856
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Syntax extensions for Parallel require Monad[M], but implementation doesn't #2254

oleg-py opened this issue May 10, 2018 · 3 comments · Fixed by #3856

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@oleg-py
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oleg-py commented May 10, 2018

I discovered today that to use syntax extension for parTraverse/parSequence, a Monad[M] instance is required.

final class ParallelTraversableOps[T[_], A](val ta: T[A]) extends AnyVal {

  def parTraverse[M[_]: Monad, F[_], B]
  (f: A => M[B])(implicit T: Traverse[T], P: Parallel[M, F]): M[T[B]] =
    Parallel.parTraverse(ta)(f)

}

While Parallel.parTraverse is defined as follows (notice lack of Monad[M])

  def parTraverse[T[_]: Traverse, M[_], F[_], A, B]
  (ta: T[A])(f: A => M[B])(implicit P: Parallel[M, F]): M[T[B]] = {
    val gtb: F[T[B]] = Traverse[T].traverse(ta)(f andThen P.parallel.apply)(P.applicative)
    P.sequential(gtb)
  }

Same is true for parSequence. Is that an oversight?
@LukaJCB
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LukaJCB commented May 10, 2018

Yes it is, unfortunately we can't change it until cats 2.0.
There's already a PR at #2180

@tnielens
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tnielens commented Apr 8, 2021

Seems like this issue missed cats 2.0 opportunity. Might be worth labeling it with 3.0.

@joroKr21
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joroKr21 commented Apr 8, 2021

Why can't we change it? Implicits can be changed in a binary compatible way by making the old ones non-implicit and adding the new ones.

@joroKr21 joroKr21 mentioned this issue Apr 8, 2021
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Remove redundant Monad constraints from Parallel syntax joroKr21/cats
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@joroKr21 @oleg-py @LukaJCB @tnielens
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