Add Constant and replace transparent parameters

[nicolasstucki]

Introduce the type Constant which is a subtype of Singleton but only allows constants (e.i. known values).

Use Constant to mark parameters of transparent methods that must be passed as constants or inlined constants.

[smarter]

Singleton is already fishy and should ideally be replaced by something else (keyword, annotation, ...), I don' think we should introduce more magic types like it unless we have a very good reason and we can prove that they respect the subtyping laws. (Singleton cannot: if A <: X and B <: X then we expected A | B <: X but 1 | 2 is not a singleton).

[smarter]

Maybe @Blaisorblade can tell us how bad this would be :)

[Blaisorblade]

Why this change?

Using a type for Singleton is already fishy. If A <: Singleton and B <: Singleton, it follows that A | B <: Singleton, but for instance 1 | 2 is not actually a singleton type. The same applies here: if A <: Constant and B <: Constant then A | B <: Constant. And neither is easy to special-case, since you can write e.g.

def f[Bound, A <: Bound, B <: Bound](a: A, b: B): C = if (...) a else b

And if you manage to make it work, you'll have recreated a way to check programs which is syntax-directed, not type-directed.

Constant seems to also have a more basic problem: since 1: Constant, then it should be possible to write

val x: 1 & Constant = 1
x: Constant // should work

but your examples suggest otherwise.

EDIT: LOL, ninjaed by @smarter.

[Blaisorblade]

I'm concerned about the design, see objections in comment and by @smarter.

[Blaisorblade]

We talked about this also with Martin today, and we agreed to think harder about it. He agreed Singleton is problematic, but actually Constant is muddier. Singleton clearly breaks because it describes types; but Constant describes "compile-time values", and it seems to me that Int & Constant resembles a bit quoted.Expr[Int].

[nicolasstucki]

I agree. The main idea was to express constantness of a parameter of an inlined method (for macros). As singleton already covered constants I explored this design that clearly has issues.

[nicolasstucki]

To be more precise Int & Constant would represent a quoted.LiftedExpr[Int], one of the implementations of quoted.Expr in which the quoted code is represented as its runtime value.

[nicolasstucki]

We should first solve the issue with Singleton