Proposal to Add Implied Instances to the Language

Scala Improvement Process
95

I think original proposal that omits assignment operator in typeclass instance definition has some drawbacks.

  1. Implied instance definition looks almost the same as implied alias definition:
implied instanceDefinition for A[B] { ... }
implied aliasDefinition for A[B] = { ... }

Only one non-blank char of difference. Just like with method returning Unit and method with inferred types:

def methodReturningUnit { ... }
def methodWithInferredReturnType = { ... }

Notation for methods without assignment operator was dismissed on many reasons and the imposed alternative is to replace def method { ... } with def method: Unit = { ... } which is 6 non-blank characters longer. Remember that methods returning Unit are quite frequent.

  1. Conflict between abstract implied instances and short class notation.

Objects and classes definitions can omit curly braces if there’s nothing to add or change in object / class definition, i.e. we can write:

class A // {} not needed
class A extends B // {} not needed
object A // {} not needed
object A extends B // {} not needed

If implied instances notations are analogous to objects and classes notations then following lines would mean concrete definitions, not abstract members:

implied A
implied A given B

But then how to define abstract implied instances?

abstract class TypeclassesAggregate {
  implied A // does not look like abstract member
  abstract implied B // well, here 'B' refers in fact to method name, but methods don't need 'abstract' modifier
}

If we always require = (assignment operator) in implicit instances definitions then above problems don’t exist or are reduced.