[e-lang] Returning guards from __getAllegedType: opinions needed

[Mark S. Miller]

Kevin Reid wrote:
> A principle which accounts for both of these is that anything between  
> the 'def' and the method bodies --
> 
>    def foo extends bar { to run(arg :guard) :int { return 99 } }
>        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> 
> is public. 
["implements baz" removed from the example so that email clients won't rewrap. 
Kevin qualifier about the "implements baz" part remains true.]

 > If the programmer wishes to hide a powerful guard, or a more specific
 > guard, they can re-coerce inside the body:
 >
 >    def f(box :Box) {
 >      def thing :Unseal := box
 >    }


The thing I really like about this rule is that we can explain what's hidden 
according to the syntactic placement of curly brackets, which fits well with 
the rest of the E language.

This let us sensibly also reserve the right for __getAllegedType to reveal 
other information appearing between the "def" and the method/matcher bodies, 
such as the ASTs of the patterns. I can imagine this being useful, e.g., for 
future documentation generator tools. For example, in E one defines a 
variable-arity function as

     ? def add {
     >     match [`run`, args] {
     >         var result := 0
     >         for arg in args {
     >             result += arg
     >         }
     >         result
     >     }
     > }
     # value: <add>

     ? add(3,4,5)
     # value: 12

     ? add.__getAllegedType().help()
     # value: interface "__main$add" {
     #        }
     #

It would be nice if, in a future version of E, add's alleged type explained 
its protocol better. It would therefore be good for us to reserve the right 
for future __getAllegedType behavior to be enhanced to provide such 
information. We allow this merely by saying that the programmer may not rely 
on future __getAllegedType behavior not revealing information visible outside 
the curlies which bracket the object's method/matcher bodies.

-- 
Text by me above is hereby placed in the public domain

     Cheers,
     --MarkM