Re: Scheme BNF

[Kent Dybvig <dyb%indiana.csnet@CSNET-RELAY.ARPA>]

    Questions:

Answers and more questions:
    
    - Does anyone object to defining (COND) and (CASE) to return unspecified
    values?  I think this would be a good idea.

I thought that COND and CASE with no match and no ELSE were already defined
to return unspecified values.  If not, I think this should be the definition.

Can we also have (BEGIN) return an unspecified value instead of being invalid
syntax?
    
    - Shouldn't backquote work on vectors?  If not, then what's the rationale
    for making them work on lists and not vectors?

I think it should not work on vectors.  It makes sense to me only in non-
constants, and until we make a firm statement that vectors have some useful
meaning in Scheme other than as constants, backquote should not work within
vectors.  I also have some amount of hesitance about splicing a list into
a vector, etc., and what that would mean.
    
    - What kinds of constants should be allowed in CASE clauses?  I don't
    really care, but I can imagine two possible answers:
    
    1. If EQV? is used ...
    
      <case clause> ::= ((<casable>*) <sequence>)
      <casable> ::= <boolean> | <number> | <character> | <symbol>
    
    2. Use EQUAL? for comparisons,  ...
    
      <case clause> ::= ((<datum>*) <sequence>)

I do not like the use of EQUAL?.  If you don't like <casable>, how about
<atom>?  Seriously, we need some term anyway, since it is the same thing
accepted by EQV?.  What about <simple>?  Or <tag>?)

Also, it's been said before, but I think we should allow for the optional
syntax of a single "<casable>" not in a list.

      <case clause> ::= (<datum> <sequence>)
    
    - I definitely want like to flush the (<var>) syntax for iteration specs in
    DO.  It is inconsistent for DO to have this alternate form and not LET,
    and I don't like either.  Does anyone actually ever use it?

    - I would also like to flush the (<var> <init>) syntax for DO iteration
    specs.  Does anyone ever use this?  I always wrap the DO in a LET if I
    need extra bindings.  No replies from people who don't use DO, please.
    
I agree completely with your sentiments, and I do use DO.

    - Oh yeah, I forgot to mention in my previous message that I would like to
    document DELAY and FORCE, as non-essential special form and procedure,
    respectively.  I believe they're non-controversial, and desirable given
    that they have a central role in S&ICP and are a little tricky to
    describe and implement.
    
If you don't mind, please send out your prose for these and include the
appropriate code for them.  I don't have them in Chez Scheme yet and I'm
not in the mood to think.


Picky comment:

I am uncomfortable with the use of <sequence> in light of the existence
of sequence as a special form.  Furthermore, I think that <expression>+
would be more expressive where it appears, and in Scheme there should be
no confusion with the operator + or sign +.