Dotty and types:
the story so far

                                    Guillaume Martres - EPFL

Takeaway message

                     Hacking compilers is fun and profitable!

Typing example

def foo[T](x: T): T = x

foo(1) // How is this typed?
              

1. Typing foo creates a type variable T?

foo[T?]
              

2. Typing foo[T?](1) adds a constraint to T?

foo[T?](1) // T? >: Int
                

3. Instantiating T? = Int respects the constraints

foo[Int](1)
                

Safe instantiation

def foo[T](…): RetType = …
foo[T?](…) // T? >: Lo <: Hi
  .bar(…)
                

• Can we instantiate T? right after typing foo[T?](…) ?
• It depends on where T appears in RetType

T appears	Safe?	T? =
nowhere	Yes	Lo (arbitrarily)
covariantly	Yes	Lo
contravariantly	Yes	Hi
co- and contravariantly or invariantly	No	¯\_(ツ)_/¯