immutable.Seq missing semigroup

[JosephMoniz]

Looks like I missed adding a semigroup instances on immutable.Seq.

Before:

scala> import cats._, cats.syntax.all._, scala.collection.immutable.Seq
import cats._
import cats.syntax.all._
import scala.collection.immutable.Seq

scala> Seq(1, 2, 3) |+| Seq(3, 4, 5)
<console>:19: error: value |+| is not a member of scala.collection.immutable.Seq[Int]
       Seq(1, 2, 3) |+| Seq(3, 4, 5)

After:

scala> import cats._, cats.syntax.all._, scala.collection.immutable.Seq
import cats._
import cats.syntax.all._
import scala.collection.immutable.Seq

scala> Seq(1, 2, 3) |+| Seq(3, 4, 5)
res0: scala.collection.immutable.Seq[Int] = List(1, 2, 3, 3, 4, 5)

See: #3622

[codecov-commenter]

Codecov Report

Merging #3624 into master will decrease coverage by 0.01%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master    #3624      +/-   ##
==========================================
- Coverage   90.31%   90.30%   -0.02%     
==========================================
  Files         391      391              
  Lines        8869     8870       +1     
  Branches      254      249       -5     
==========================================
  Hits         8010     8010              
- Misses        859      860       +1     

[djspiewak]

I do think we need to talk a bit about source compatibility though and 2.12 support. These instances basically don't resolve on 2.12 because Seq is a different type. Is that something which is worth addressing?

[JosephMoniz]

All of these instances resolve on 2.12 just fine as long as you reference scala.collection.immutable.Seq and not scala.collection.mutable.Seq. The downside here is that in order to use any of these typeclasses in 2.12 or earlier you need to more explicitly reference immutable.Seq.

Note: My example in the original PR description is running in 2.12

In terms of source compatibility though I think this is fine for 2.12 because unless you're going out of your way to convert mutable.Seq into immutable.Seq then these changes should have no impact.

[JosephMoniz]

Added some additional thoughts on source parody here: #3622 (comment)