Welcome to OGeek Q&A Community for programmer and developer-Open, Learning and Share
Welcome To Ask or Share your Answers For Others

Categories

0 votes
1.1k views
in Technique[技术] by (71.8m points)

scala - What is the idiomatic way to pattern match sequence comprehensions?

val x = for(i <- 1 to 3) yield i
x match {
    case 1 :: rest => ... // compile error
}

constructor cannot be instantiated to expected type; found : collection.immutable.::[B] required: scala.collection.immutable.IndexedSeq[Int]

This is the same problem as MatchError when match receives an IndexedSeq but not a LinearSeq.

The question is, how to do it right? Adding .toList everywhere doesn't seem right. And creating an own extractor which handles every Seq (as described in the answer of the other question) would lead to a mess if everybody would do it...

I guess the question is, why can't I influence what the return type of sequence comprehensions is, or: why isn't such a generalized Seq extractor part of the standard library?

See Question&Answers more detail:os

与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…
Welcome To Ask or Share your Answers For Others

1 Reply

0 votes
by (71.8m points)

Well, you can pattern-match any sequence:

case Seq(a, b, rest @ _ *) =>

For example:

scala> def mtch(s: Seq[Int]) = s match { 
  |      case Seq(a, b, rest @ _ *) => println("Found " + a + " and " + b)
  |      case _ => println("Bah") 
  |    }
mtch: (s: Seq[Int])Unit

Then this will match any sequence with more than (or equal to) 2 elements

scala> mtch(List(1, 2, 3, 4))
Found 1 and 2

scala> mtch(Seq(1, 2, 3))
Found 1 and 2

scala> mtch(Vector(1, 2))
Found 1 and 2

scala> mtch(Vector(1))
Bah

与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…
OGeek|极客中国-欢迎来到极客的世界,一个免费开放的程序员编程交流平台!开放,进步,分享!让技术改变生活,让极客改变未来! Welcome to OGeek Q&A Community for programmer and developer-Open, Learning and Share
Click Here to Ask a Question

...