[Experimental] Minimal support for universally quantified values.

[TomasMikula]

In type position,

∀[A => F[A]]

is rewritten to structural type

{ def apply[A](): F[A] }

In term position,

∀[A => F[A]](fa)

is rewritten to

new { def apply[A](): F[A] = fa }

Example

val right = ∀[A => Either[A, Int]](Right(42))
val e: Either[String, Int] = right[String]

See more examples in tests.

Shortcomings / TODOs

Reflection (structural types)

Invocation of a method on a structural type uses reflection.

Defining a supertype like

trait ForAll[F[_]] {
  def apply[A](): F[A]
}

would restrict the type constructors to the kind * -> *, so I went with structural types. Ideas on how to both support arbitrary kinds and avoid reflection are welcome.

Nested ∀s

Nested ∀s don't work yet:

type Exists[F[_]] = ∀[B => ∀[A => F[A] => B] => B]

gives not found: type ∀ for the inner ∀.

Syntax

It would be nice if instead of

∀[A => List[A]]

one could write

∀[List]

It is unlikely this will be made possible, since information about List's type parameters is absent in this syntax tree.

I can imagine this to be supported instead:

∀[List[?]]

[paulp]

Forall1, Forall2, etc is better than structural types. I think structural types are a non-starter.

[TomasMikula]

Closing in favor of #54.