GenCodec for Seq creates a Vector instead of List when reading

[ghik]

Originally reported in #62

Technically, this is not a bug - GenCodec.read[Seq[A]] is free to choose whatever implementation of Seq it wants. However, this is non-intuitive because the standard Seq(...) factory creates a List by default.

It seems that the problem stems from an inconsistency in Scala collections library. GenCodec.seqCodec[Seq,A] is based on implicit instance of CanBuildFrom[Nothing,A,Seq[A]] which, for whatever reason, creates a Vector ¯\_(ツ)_/¯. This quirk can manifest in simpler examples, e.g.

scala> Seq(1,2,3)
res19: Seq[Int] = List(1, 2, 3)

scala> Seq(1,2,3).to[Seq]
res18: Seq[Int] = Vector(1, 2, 3)

So I'm not sure what to do about it yet, because it's not clear to me what would be the "correct" behaviour here. If we patched GenCodec specifically for Seq then we'd have to look for all other possible similar inconsistencies in the collections library and patch them accordingly...

[plokhotnyuk]

Definitely it would be better to patch CanBuildFrom[Nothing,A,Seq[A]] in the Scala repository.

Since 2.13.0-M4 version that trait was replaced by BuildFrom and this inconsistency was fixed:

andriy@notebook:~$ /opt/scala-2.13.0-M4/bin/scala
Welcome to Scala 2.13.0-M4 (Java HotSpot(TM) 64-Bit Server VM, Java 1.8.0_171).
Type in expressions for evaluation. Or try :help.

scala> Seq(1, 2, 3)
res0: Seq[Int] = List(1, 2, 3)

scala> Seq(1, 2, 3).to(Seq)
res1: Seq[Int] = List(1, 2, 3)

[ghik]

Fixed in 2.0.0-M1 release for Scala 2.13. Won't fix for Scala 2.12

[ghik]

Ok, after a second thought it's ugly to have different behaviour in Scala 2.12 and 2.13. Fixed in cfd16b5