DSL generics

[MeAmAnUsername]

Generics for PIE DSL
Full generics cannot be implemented in NaBL2, but some parts can.
This proposal aims to implement generics for data types.

use cases

• foreign functions may return generics, e.g. JSGLR1ParseResult.getAst() returns an Optional<T>
  • in particular, spoofax 3 tasks return Result<T, E>
• Create generic functions, e.g. assertEquals(expected: T, actual: T, message: string) -> unit

syntax

New type: Type.GenericTy = [[ID]<{[ID] ", "}+>]. Exactly the same as Java, e.g. Result<T, E>. Can be used everywhere a type is used.

Functions gain the ability to specify generic type parameters:
func createOk<T: Object>(val: T) -> Result<T, Exception> = foreign java mb.common.result.Result#ofOk
func id<T>(val: T) -> T = val

Call a generic function:
val result = createOk<Foo*>([createFoo()]);
Requirement to explicitly add types may be lifted in the future (presumably requires statix analysis though).

Static semantics

For now, data Foo<Bar>: Bar must be a valid data type, it is not a generic data type.
Later: allow data Foo<T: Language>. If the generic type parameter adds a supertype it is a generic parameter, without a parameter it is specific. Allow T: _ to use TopTy as parameter.
Even later: deprecate data Foo<T>. After deprecation, data Foo<T> becomes syntactic sugar for Foo<T: _>
func createOk<T: Object>(...)

Code generation:

[todo]

Issues

[todo]

[MeAmAnUsername]

Trying this today to see if it works; otherwise this will be deemed impossible until static semantics are implemented in Statix

[MeAmAnUsername]

Status update: implementing static semantics in Statix since end of November. Currently still translating existing semantics, but it seems likely that generics can easily be implemented with Statix.

[MeAmAnUsername]

Clarification on previous comment: I expect that generics will be hard, but mostly due to the inherent complexity of generics, not due to the incidental complexity of Statix

[MeAmAnUsername]

Blocked by ambiguity in #208

[MeAmAnUsername]

There is an implementation to resolve #208 (syntax was changed from Foo<T> to Foo[T]) so this issue is now unblocked again.

Still to do: add a few compiler tests so that there is some protection against regressions.

[MeAmAnUsername]

Blocked again by syntax issues, this time by #237.

[MeAmAnUsername]

Implemented, currently opening PR