Using constructor where function expected

Question

Having two simple classes taking Int as an argument:

case class Foo(i: Int)
     class Bar(j: Int)

I can say:

List(1,2,3) map Foo

Which works fine and is equivalent to a bit more verbose:

List(1,2,3) map {Foo(_)}

However Bar (because it is not a case class?) cannot be used in the same construct:

List(1,2,3) map Bar

  error: not found: value Bar
          List(1,2,3) map Bar
                          ^

Is there some special syntax to reference any constructor and take advantage of eta expansion? List(1,2,3) map {new Bar(_)} seems a bit more verbose compared to Foo.

Eta expansion is something that happen to methods. Note that a constructor is not a method. — Daniel C. Sobral
– Daniel C. Sobral, Commented Oct 8, 2011 at 13:56
@AshkanKh.Nazary A constructor is a sequence of instructions that initializes a newly allocated object. — Daniel C. Sobral
– Daniel C. Sobral, Commented Dec 2, 2019 at 18:44
@DanielC.Sobral And that is not a method or a function ? You can't call it explicitly or pass it around or apply eta expansion on it to turn it into a function ? — Ashkan Kh. Nazary
– Ashkan Kh. Nazary, Commented Dec 4, 2019 at 9:41
@AshkanKh.Nazary No, you can't. Try new String _, for example. It is also different at the bytecode level, and there's all sort of differences in JVM expectations between methods and constructors. — Daniel C. Sobral
– Daniel C. Sobral, Commented Dec 19, 2019 at 17:50

missingfaktor · Accepted Answer · 2011-10-08 08:20:26Z

12

It works in the former case, because companion object of a case class extends appropriate FunctionN trait. (object Foo extends (Int => Foo) in your example.) For non-case classes, you could do this manually:

scala> class Bar(i: Int)
defined class Bar

scala> class Bar(i: Int); object Bar extends (Int => Bar) { def apply(i: Int) = new Bar(i) }
defined class Bar
defined module Bar

scala> List(2, 3) map Bar
res17: List[Bar] = List(Bar@1f99e90, Bar@1191056)

IMO it's better to go with new Bar(_) as this extra boilerplate might not be worth the little concision achieved.

answered Oct 8, 2011 at 8:20

missingfaktor

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2 Comments

Peter Schmitz Over a year ago

Interesting, wasn't aware that a case class extends FunctionN. I remember a SO answer where all automatic features of case classes are listed but don't remember that issue mentioned.

missingfaktor Over a year ago

case classes don't. Their companion objects do.

Theo · Accepted Answer · 2011-10-08 08:30:52Z

2

Since Foo is a case class there is also a companion object called Foo, which implements the Function1 interface (had the constructor taken three arguments it would have been the Function3 interface).

answered Oct 8, 2011 at 8:30

Theo

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