Cannot call use class name directly to map type after adding a companion project

Question

When we have a case class, we call map the type with the name of the type, e.g.:

case class Foo(value: String)
val value = Some("Yay!")
val foo = value.map(Foo)

But if we also provide a companion object, this stops working value.map(Foo) and looks like this works: value.map(Foo(_)). Why?

case class Foo(value: String)
object Foo {}
val value = Some("Yay!")
val foo = value.map(Foo)
println(foo)

ScalaFiddle.scala:5: error: type mismatch;
found : ScalaFiddle.this.Foo.type
required: scala.this.Function1[lang.this.String,?]
val foo = value.map(Foo)

I just paste the code in the RELP and it worked (scala 2.12.7), which Scala version are you using? or can you provide a more complete example that does fail? — Luis Miguel Mejía Suárez
@LuisMiguelMejíaSuárez I just tried it in a worksheet and it doesn't work: scalafiddle.io/sf/nXeDmV1/0 - what did you do to make it compile? — James Whiteley
@LuisMiguelMejíaSuárez, code added, and we're running 2.11.8. — Yuchen

Andrey Tyukin Andrey Tyukin · Accepted Answer · 2018-11-29T15:33:39

If you don't define object Foo at all, then the synthetic companion object has the following declaration:

<synthetic> object Foo 
   extends scala.runtime.AbstractFunction1[String,Foo] 
      with Serializable

But if you define your own object Foo as follows

case class Foo(v: String)
object Foo {}

then the declaration of the Foo object changes accordingly to:

object Foo extends scala.AnyRef with Serializable

and it no longer extends Function1.

The method apply(v: String): Foo is still automatically generated on Foo, but it no longer implements the Function1[String, Foo] interface. If you declare the companion object like this:

object Foo extends (String => Foo) { ... }

then you again can use it in expressions like value.map(Foo).

The value.map(Foo(_)) syntax always works, because it's just a shortcut for

value.map(x => Foo.apply(x))

and the closure doesn't care at all about what interfaces are implemented by Foo, it cares only about the existence of the apply method.

Cannot call use class name directly to map type after adding a companion project

1 Answers