Writer can be covariant?

[windymelt]

I found Writer[L, V] is currently invariant.

cats/core/src/main/scala/cats/data/WriterT.scala

Line 28 in 824b1cb

finalcaseclassWriterT[F[_], L, V](run: F[(L, V)]){

Can it be covariant?

• ZIO
  • This is not code itself, but they accepts treating Writer as covariant.
• Scalaz
  • Currently not treating Writer as covariant.
• Meanwhile, Reader[-R, A] is contra-variant on parameter R in Cats.

I'm newbie to contribute Cats, so any editing/closing is welcome.

Example that we want covariance for Writer

importcats.data.WritertypeWords[A] =Writer[List[String], A] traitLongCat[F[_]]:defthe:F["the"] deflong(expr: F["the"|"long"]):F["long"] defcat(expr: F["long"]):F["cat"] endLongCatobjectLongCatImplextendsLongCat[Words]:defthe:Words["the"] =Writer(List("the"), "the") deflong(expr: Words["the"|"long"]):Words["long"] = expr.mapBoth((l, v) => ("long":: l, "long")) defcat(expr: Words["long"]):Words["cat"] = expr.mapBoth((l, v) => ("cat":: l, "cat")) endLongCatImpl locally:importLongCatImpl.*valrepr= cat(long(long(the))) // => Found: Words[("the" : String)]// Required: Words[("the" : String) | ("long" : String)]  println(repr.run._1.reverse) // What I want: "the" :: "long" :: "long" :: "cat" :: Nil

[johnynek]

It can't be covariant, because we don't know anything about F[_]. We could require F[+_] which would allow it to be covariant, but that was not done.

that said, note that def widen[B >: A]: F[B] exists on Functor[F] and internally that can always be implemented by returning the same item (or at worst a safe cast due to the functor laws). So, you should be able to do fa.widen when you need covariance.

[satorg]

Hey everyone, just to check the status: is there anything we are going to do with this draft?