The syntax of implicits

[kyouko-taiga]

We discussed the syntax of implicit parameters with @dabrahams and @camio. It's been rightly pointed out that using ? is a bad idea because the symbol is already strongly associated with optionals. We then came up with the following idea to replace this syntax:

• Implicit values are introduced with a modifier on binding declaration spelled implicit, replacing the current @given annotation.
• Implicit parameters are introduced by adding implicit after the colon in parameter lists.

For example:

fun describe<C: Collection>(_ p: C.Position, in context: implicit let C) {
  context[p].description()
}

public fun main() {
  implicit let things: Array = [1, 2, 3]
  var x = things.start_position()
  describe(x)
}

To support these choices I think we should also require that:

• implicit become a keyword to let the parser identify the beginning of a declaration without lookahead.
• an implicit parameter can't be introduced in a lambda without specifying its type (e.g., fun f(x: implicit)).