Idiomatic way to convert A => Seq[B]

Question

I'd like to convert a single value into a collection of multiple "characteristics", without using a mutable data structure to collect the values. I'd like something like this fantasy construct which uses pattern matching, but does not stop after first match:

scala> 2 multimatch {
  case i if i > 0 => "Positive"
  case i if i < 0 => "Negative"
  case i if (i % 2 == 0) => "Even"
  //yadda yadda
}
res0: Seq[java.lang.String] = List(Positive, Even)

Only one case can match. You need to nest them so 2 can be recognized as positive and even, or am I missing the point here? — agilesteel, Jul 16 '11 at 21:07
Yeah, I'm describing something similar to pattern matching, where > 1 matches can be returned. — Adam Rabung, Jul 16 '11 at 21:15

score 21 · Accepted Answer · answered Jul 16 '11 at 22:27

21

Using the pimp pattern, partial functions and repeated parameters, this is how close I can get:

class MultiMatcher[A](a: A) {
  def multiMatch[B](pfs: PartialFunction[A, B]*): Seq[B] = {
    pfs.map(_.lift(a)).collect{ case Some(v) => v } 
  }
}

implicit def toMultiMatcher[A](a:A): MultiMatcher[A] = new MultiMatcher(a)

2 multiMatch (
  { case i if i > 0 => "Positive" },
  { case i if i < 0 => "Negative" },
  { case i if i % 2 == 0 => "Even" }
)
// returns Seq[java.lang.String] = ArrayBuffer(Positive, Even)

answered Jul 16 '11 at 22:27

huynhjl

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Why not `pfs.flatMap(_.lift(a))`? – Gordon Gustafson Apr 01 '16 at 15:40
Implicit conversions are now 'opt-in', so add `import scala.language.implicitConversions` before defining the implicit conversion. – Gordon Gustafson Apr 01 '16 at 15:45

score 8 · Answer 2 · answered Jul 16 '11 at 21:24

First you define your characteristics as functions from Int to Option[String]

val pos = (i:Int) => if (i > 0) Some("Positive") else None
val neg = (i:Int) => if (i < 0) Some("Negative") else None
val even = (i:Int) => if (i % 2 == 0) Some("Even") else None

Then you make a list of characteristics.

val characteristics = pos::neg::even::Nil

And then you use flatmap to get a list of those characteristics that apply to a certain object.

scala> characteristics.flatMap(f=>f(2))
res6: List[java.lang.String] = List(Positive, Even)

anrizal - Anwar Rizal · Answer 3 · 2011-07-17T00:01:39.150

First, define a multimatch function as follows:

scala> def multimatch[A,B]( value : A,ps: ( A => Boolean, B)*) =
     |           for ( p <- ps
     |                 if (p._1(value))) yield p._2
multimatch: [A,B](value: A,ps: ((A) => Boolean, B)*)Seq[B]

Then, here we go:

    scala> multimatch(2,
         |           ( ((x :Int) => x > 0) -> "POSITIVE"),
         |           ( ((x :Int) => x < 0) -> "NEGATIVE"),
         |           ( ((x :Int) => x % 2 == 0) -> "EVEN")
         |       )

res4: Seq[java.lang.String] = ArrayBuffer(POSITIVE, EVEN)

Or, less cluttered:

scala> multimatch(2,
     |           ( (x :Int) => x > 0 , "POSITIVE"),
     |           ( (x :Int) => x < 0, "NEGATIVE"),
     |           ( (x :Int) => x % 2 == 0, "EVEN")
     |       )
res5: Seq[java.lang.String] = ArrayBuffer(POSITIVE, EVEN)

Idiomatic way to convert A => Seq[B]

3 Answers3

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