Why is VectorBuilder in the package scala.collections.immutable? -

vectorbuilder defined in same source file vector. vector immutable , in scala.collections.immutable package, consequence builder in same package.

as far can tell, canbuildfrom uses vectorbuilder default, if return type not explicitly typed.

• is there reason not having builder in separate file in mutable package?
• is builder not meant used directly? if so, builder or buffer used create seq?

vectorbuilder not meant used directly. if want builder vector, need call vector.newbuilder[t], returns builder[t, vector[t]] (with underlying instance being vectorbuilder).

so if want default builder used create seq, need call seq.newbuilder:

scala> seq(1,2,3) res0: seq[int] = list(1, 2, 3)  scala> seq.newbuilder[int] res1: scala.collection.mutable.builder[int,seq[int]] = listbuffer()  scala> seq.newbuilder[int].result res2: seq[int] = list() 

the above shows default implementation of seq list, and, logically, default builder seq mutable.listbuffer.

listbuffer more list builder, that's why in collection.mutable whereas vectorbuilder not buffer, cannot used else other build vector. that's why defined locally in vector. not sure why isn't private, cannot see referenced anywhere in public api of vector itself. maybe should (private).

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just reference, there no hidden magic happening canbuildfrom, goes through newbuilder above:

1. when not specify expected collection type, in seq(1,2).map(_+1), available canbuildfrom comes from companion object seq, , of type canbuildfrom[seq[_], t, seq[t]]. means result seq too.

2. like companion objects of collections, canbuildfrom instance seq provides 1 thing: call seq.newbuilder (that's defined in gentraversablefactory ...)

that's why canbuildfrom vector uses vectorbuilder. example, in this:

scala> vector(1,2,3).map(_+1) res12: scala.collection.immutable.vector[int] = vector(2, 3, 4) 

the builder used is:

scala> implicitly[canbuildfrom[vector[int], int, vector[int]]].apply() res13: scala.collection.mutable.builder[int,vector[int]] =    scala.collection.immutable.vectorbuilder@43efdf93