[R6RS] Record proposal syntax simplification

[Anton van Straaten]

>>However, the keywords have a low signal-to-noise ratio, since they 
>>communicate only a single bit of information that's already present in 
>>another form.  They also tend to obscure more important information,
>>...
> 
> 
> True.  Since it's only a single bit of information, we could use ! instead
> of immutable and nothing in place of mutable.
> 
>    ((! x get-x set-x!) init)
>    ((x get-x) init)
> 
> The ! won't obscure much.

I am happier with this than with the longer keywords.

I don't like that the two-element grouping (x get-x) changes to a 
four-element one, (! x get-x set-x!), in the mutable case.  I think it 
would be easier to remember the syntax if there were more symmetry 
between the cases, which was something I was going for in my syntax.

However, that's addressed to some extent by this:

> How about:
> 
>    (<spec> init)
>    (! <spec!> init)
> 
> where
> 
>    <spec> -> <id> | (<id> <accessor>)
>    <spec!> -> <id> | (<id> <accessor> <mutator>)

So the earlier examples would become:

    (! (x get-x set-x!) init)
    ((x get-x) init)

That's a bit better, IMO.  I still find the injection of ! to be 
slightly off-putting, and since convention dictates that mutator names 
contain bangs anyway, it doesn't seem necessary to me.  However, I could 
live with it.

>>3. Optional init expressions
>>----------------------------
> 
> 
>>In the case where a field without an init expression also has no 
>>corresponding formal with the same name, my preference is to make this 
>>an error. This will force all fields to be properly initialized, either 
>>explicitly via an init expression, or implicitly via the constructor 
>>arguments.
> 
> 
> I agree.

If init expressions are made optional in this way even in the explicit 
naming layer, then combined with the syntax defined above, I'd be pretty 
happy with the proposal.

Anton