Multiple assignment of non-tuples in scala

Question

Just to clarify, when I say multiple assigment, parallel assignment, destructuring bind I mean the following pattern matching gem

scala> val (x,y) = Tuple2("one",1)
x: java.lang.String = one
y: Int = 1

which assigns "one" to x and 1 to y.

I was trying to do

val (x,y) = "a b".split()

I was expecting that scala would attempt to pattern match the array with the pattern, and would throw a runtime exception if the length of the array wouldn't match the length of the pattern.

All my attempts to easily convert an Array to a Tuple2 were futile.

scala> Tuple2(Array(1,2):_*)
<console>:7: error: wrong number of arguments for method apply: (T1,T2)(T1, T2)
in object Tuple2
       Tuple2(Array(1,2):_*)
       ^

scala> Tuple2(Array(1,2).toList:_*)
<console>:7: error: wrong number of arguments for method apply: (T1,T2)(T1, T2)
in object Tuple2
       Tuple2(Array(1,2).toList:_*)

Is there any neat way to use multiple assignment with arrays or lists?

Solution 1

All you need to do is make your val side (left of the =) compatible with your initializer (right of the =):

scala> val Array(x, y, z) = "XXX,YYY,ZZZ".split(",")
x: java.lang.String = XXX
y: java.lang.String = YYY
z: java.lang.String = ZZZ

As you expected, a scala.MatchError will be thrown at runtime if the array size don't match (isn't 3, in the above example).

Solution 2

Since your string can have arbitrary contents, the result cannot be guaranteed to have a 2-tuple-form by the type-system (and no conversion would make sense at all). Therefore you'll have to deal with sequences (like arrays) anyway.

Thankfully there are right-ignoring sequence patterns which allow you to match the result values conveniently nevertheless.

val Seq(x, y, _ @ _*) = "a b".split(" ")

Solution 3

scala> val Array(x, y, _*) = "a b" split " "
x: java.lang.String = a
y: java.lang.String = b

Comments

[Elazar Leibovich]

Quoting myself: "I was expecting that scala would attempt to pattern match the array with the pattern, and would throw a runtime exception if the length of the array wouldn't match the length of the pattern", also see Randall's solution. Anyway this is a nice one as well. So simple I wish I thought about it myself. +1.

[Randall Schulz]

In Scala 2.8, Array is no longer Seq, so this technique doesn't work there.

[retronym]

This works because the object Array includes the unapplySeq method, making it usable as an Exractor Pattern. Array.unapplySeq("XXX,YYY,ZZZ".split(",") is called, returning Some(ArrayBuffer(XXX, YYY, ZZZ)), which is a Some, and contains three elements to bind to x, y, and z.

[psp]

The technique certainly works, you just write "Array" instead of "Seq". Or if you can be chill about the end of the String, val Array(x, y) = "a b" split " " take 2

[Blaisorblade]

val Array(x, y, _*) = "a b".split(" ") also works with less syntactic clutter. I don't think that _ @ is ever useful.

[Batato]

why val Array(X, Y, Z) = "XXX,YYY,ZZZ".split(",") doesn't work? I get <console>:14: error: not found: value X...

[Jim Balter]

@Batato match variables must be lowercase