classes / enums / interfaces / services / serviceinterfaces
===========================================================

See also `class_kinds_keywords.txt`

The kinds of types could be:

    classes
    enums
    interfaces
    services
    service interfaces

There are some constraints:

* enums can have a constructor, but it can only be accessed locally to
  instantiate the enum values.
* interfaces cannot have constructors or fields
* service interfaces might have abstract constructors, but not fields
* services might have a specific constructor (or just an optional
  zero-argument constructor?)

Declaration ordering constraints:

* Fields must already come before functions/constructors
* Require constructors to come before functions?
* Require the following to come before anything else (anything else would
  implicitly be considered to be a class that doesn't implement any
  interfaces):
    - enum values
    - class/service "implements" lines
    - interface/service interface specifiers
* Don't require any particular ordering of service entry points?

Should it be required to declare exported services/entry's?
See also `main_startup.txt`


Syntax test
-----------

Class, normal:

    int x
    int y

Class, implementing an interface:

    implements SomeInterface

    impl to_string
    returns
        String s
    code
        ...
    end

Enum:

    typekind enum

    values
        # `new` could be implicit, unless another constructor is specified
        red    = 255   0   0
        green  =   0 255   0
        yellow = 255 255   0
        blue   =   0   0 255
    end

Interfaces

    typekind interface

    func to_string
    returns
        String s
    end

Services

    # TODO more inuitive syntax/keywords than `typekind service ...`
    # TODO should maybe have mulitple lines
    typekind service CLIApp
    # or this:
    servicetype CLIApp
    # or this: (best?)
    be CommandMain

    # TODO more inuitive syntax/keywords than `inject`
    inject CLIParams params

    entry main
    code
        ...
    end

Service Interfaces

    typekind serviceinterface

    # TODO more inuitive keyword than `injectable`
    injectable CLIParams params

    entry main
    end