Generic type alias

[soutaro]

This PR adds the key feature for RBS 1.8, generic type alias!

You can now define generic type aliases like:

type list[T] = [T, list[T]] | nil  # Defines linked list of `T`

type int_list = list[Integer]      # List of Integer
type object_list = list[Object]    # List of Object

We also provide new DefinitionBuilder#expand_alias variants to unfold a type alias: #expand_alias1 and #expand_alias2.

builder.expand_alias2(list, [integer])   # Unfold type alias one step with application => [integer, list[integer]] | nil
builder.expand_alias1(list)              # Unfold type alias one step with `untyped` type arguments => [untyped, list[untyped]] | nil
builder.expand_alias(list)               # Raises a runtime error because `::list` type requires one type argument

So, the type checkers will continue working if no generic type alias is used in the application. Or, you can just replace your #expand_alias calls with #expand_alias1 for the minimal support.

Generic type alias also supports variance annotations.

# The type parameter `T` is covariant: list[Integer] <: list[Object]
type list[out T] = [T, list[T]] | nil

One limitation of defining generic type alias is the regularity. It prohibits polymorphic recursion. You cannot use a type alias polymorphic in its declaration.

type foo[T] = foo[Array[T]] | nil      # This is prohibited.

• Update syntax.md