While this is an interesting effort, I can't help but be squeamish about the sheer volume of JS that gets emitted (due to type checking) for what seem like small amounts of input.
const DISABLE_TYPE_CHECKING = true
at the top of your file. It's handy to have in development vs. production.
Thus, the following would compile without warning:
let double(x as Number) x * 2
let repeat(x as String) x & x
let fun(x as Number|String)
if is-number! x
double x
else
repeat x
let fail(x as Boolean)
// this should fail at compile-time
double x
I also might add a "cast", i.e. a runtime type check. I'm not sure what the best syntax for it would be, but it would act like the following:
CAST(x, Number) // validate that x is a Number or throw a TypeError
CAST(x, Number|String) // validate that x is either a number or a string, otherwise throw TypeError
CAST(x, Number, 0) // validate that x is a Number, otherwise evaluate to 0.
I would love some feedback on this, especially from the perspective of "what would I use to help me develop", not "I need my program to be 100% statically type-safe without having to do any testing".