Cats: Filter for Free Monads

[arjunswaj]

Add Filter API for Free monads. Any if condition in the for comprehension is failing due to the missing filter APIs. There is also a StackOverflow question on the same. https://stackoverflow.com/q/41659747/3711149

[arjunswaj]

Hey guys,
Any update or a workaround available? I want to use Free Monads in Prod and this would be a great feature to have.

[johnynek]

You could used FreeT with Either[String, ?] as the inner monad then implement filter on that FreeT with it just giving you a left in the case that the filter fails.

Sorry, I'm on a phone or I would write the code out here.

[arjunswaj]

Thanks. If you can post a gist/snippet, it would be great.

[johnynek]

Okay, something like:

type MyFree[M[_], T] = FreeT[Either[String, ?], M, T]

implicit class FilterFree[M[_], T](val on: FreeT[Either[String, ?], M, T]) extends AnyVal {
  def filter(fn: T => Boolean): FreeT[Either[String, ?], M, T] =
    on.flatMap { t =>
      if (fn(t)) FreeT.pure(t)
      else FreeT.liftF(Left(s"filter failed on value: $t"))
    }
}

Something like that.

[LukaJCB]

We could think about adding a generalized version of @johnynek snippet that accepts any MonadError, might be useful 🤔

Something I came up with on the quick:

def filter[E](f: A => Boolean)(e: => E)(implicit ME: MonadError[M, E]): FreeT[S, M, A] =
  on.flatMap { t =>
    if (f(t)) FreeT.pure(t)
    else FreeT.liftT(ME.raiseError(e))
  }

[johnynek]

The above variant may be useful, but can't be used with a for comprehension.

[edmundnoble]

I don't think it makes sense to include this unless it's for MonadError over Unit.

[LukaJCB]

I vote to close this, as I don't think it's really worth including.

[johnynek]

+1 to close.

On Wed, Oct 4, 2017 at 08:04 Alexander Konovalov [e-mail]
wrote:

Instead of

for {
(a, b) <- f
} yield a

you can always write

for {
ab <- f
(a, b) = ab
} yield a

which is a bit annoying but not the end of the world ;) I think that is
the use case OP had problems with.

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