Autocasting types using 'value: Type' syntax

Question

When I am reading Scala reflection tutorial. I found a syntax very wired as follows.

  import scala.reflect.runtime.universe._
  typeOf[List[_]].member("map": TermName)

So the member function takes Name type parameter, and then "map": TermName is passed into it. What does this syntax exactly mean? I am guessing it is sugar shortcut for .member(TermName("map")).

`obj : Type` means view the object as that type, you may think of it like `obj.asInstanceOf[Type]` but, first it is type safe, and second it is more powerful, for example, in this case it triggers an **implicit conversion**. — Luis Miguel Mejía Suárez, Jul 26 '19 at 22:03
@LuisMiguelMejíaSuárez Thanks for a quick reply. Do you know any reference talking about this syntax? — kevin, Jul 26 '19 at 22:08
it should be somewhere in the [specs](https://scala-lang.org/files/archive/spec/2.13/). — Luis Miguel Mejía Suárez, Jul 26 '19 at 22:14
@kevin Type ascription is mentioned in https://www.scala-lang.org/files/archive/spec/2.13/06-expressions.html — Mario Galic, Jul 26 '19 at 22:34

Mario Galic · Accepted Answer · 2019-07-26T22:25:23.633

This syntax is called type ascription:

Ascription is basically just an up-cast performed at compile-time for the sake of the type checker.

It is used here because the signature of member is

def member(name: Name): Symbol

so it is expecting input of type Name hence

typeOf[List[_]].member("map")

gives error because "map" is not Name. Providing type ascription "map": Name triggers implicit conversion

typeOf[List[_]].member(stringToTermName("map"): TermName)

however the same can be achieved with more explicit

typeOf[List[_]].member(TermName("map"))

The implicit conversion stringToTermName technique is deprecated

  /** An implicit conversion from String to TermName.
   * Enables an alternative notation `"map": TermName` as opposed to `TermName("map")`.
   * @group Names
   */
  @deprecated("use explicit `TermName(s)` instead", "2.11.0")
  implicit def stringToTermName(s: String): TermName = TermName(s)

Autocasting types using 'value: Type' syntax

1 Answers1