Missing Factory instance for scala.Stream (2.12)

[djspiewak]

Not entirely sure if this is the right place for this; I could file it under fs2, as well. Either way, it appears that there is no instance Factory[A, Stream[A]], which in turn causes some source compatibility issues in packages (such as fs2) which have migrated to collection-compat.

No, I don't make any excuses for the use of scala.Stream. Legacy code. Uggghhhh.

Anyway, is this omission intentional?

[djspiewak]

After a bit of hunting, it appears this may be a type inference issue somewhere. Stream[_] (the existential, not the type constructor) is not being considered compatible with Nothing for the purposes of locating a CanBuildFrom which can satisfy Factory's alias.

[julienrf]

Anyway, is this omission intentional?

No, it’s not.

Stream[_] (the existential, not the type constructor) is not being considered compatible with Nothing for the purposes of locating a CanBuildFrom which can satisfy Factory's alias.

Then that should be the case with all other collection types? They all define an implicit [A]CanBuildFrom[Coll, A, CC[A]], where Coll = CC[_], and CC is the collection type constructor (e.g., List).

[djspiewak]

That's what I would have suspected. I really have no idea what's special about Stream. I'll see if I can come up with a better minimization. Right now, my minimizer is using it via the to function on fs2.CompileOps.

[szeiger]

I cannot reproduce this. In the 2.12.2 REPL:

scala> import scala.collection.generic.CanBuildFrom
import scala.collection.generic.CanBuildFrom

scala> type Factory[-A, +C] = CanBuildFrom[Nothing, A, C] // same as in collection-compat
defined type alias Factory

scala> def f[A] = implicitly[Factory[A, Stream[A]]]
f: [A]=> Factory[A,Stream[A]]

scala> f[Int]
res6: Factory[Int,Stream[Int]] = scala.collection.immutable.Stream$StreamCanBuildFrom@45790cb

[szeiger]

Probably a duplicate of #137 (which I cannot reproduce in the REPL, either)