Chain in Cats built for Scala 2.12 can keep a mutable collection

[satorg]

Example (see in Scastie):

import cats.data.Chain

// Should contain lease 2 elements initially to enforce Chain.Wrap
val buff = scala.collection.mutable.ListBuffer(1, 2) 
val chain = Chain.fromSeq(buff)

// Add more elements to the source buff
buff += 3
buff += 4

println(chain)
// Chain(1, 2, 3, 4)

This is because Chain.fromSeq takes a default Seq (defined in Predef) which in Scala 2.12 is just scala.collection.Seq which in turn can be both ..mutable.Seq and ..immutable.Seq.

This snippet does not compile in Scala 2.13 and above.

[armanbilge]

Woops. We can fix this bin-compatibly by explicitly importing immutable.Seq and then adding a deprecated method using the non-qualified Seq on 2.12 only.

[satorg]

I would suggest that the deprecated method could be fixed as well. It could check whether the seq passed is mutable and if it is so then copy all its elements into an immutable seq. It would work slower for mutables (which are not supposed to be used there anyway), but would make Chain safer.