[r6rs-discuss] Use backquote in "case"

[Aubrey Jaffer]

 | From: MichaelL at frogware.com
 | Date: Fri, 10 Nov 2006 00:27:50 -0500
 | 
 | (I was interested in feedback on the following before submitting a formal 
 | comment.)
 | 
 | Would it make sense for "case" to be defined in terms of backquote rather 
 | than quote? The problem is that the use of quote makes references to 
 | constants impossible. For example, this doesn't work:
 | 
 | (define some-const 'xxx)
 | 
 | (case some-val
 |         [(some-const)
 |                 ...])
 | 
 | If "case" were defined in terms of backquote, though, this would be 
 | possible:
 | 
 | (define some-const 'xxx)
 | 
 | (case some-val
 |         [(,some-const)
 |                 ...])

In SCM (http://swiss.csail.mit.edu/~jaffer/SCM), the QASE syntax
provides this and unquote-splicing as well.

QASE enables SIMSYNCH (http://swiss.csail.mit.edu/~jaffer/SIMSYNCH)
logic designs to refer to symbolic names rather than numerical
constants, while providing control over those numerical assignments
(unlike some implementations of enumerations).

I suspect that QASE can't be defined using just R5RS-macros (SCM does
DEFMACRO).  Is it possible using R6RS macros?

 -- Special Form: qase key clause1 clause2 ...
     `qase' is an extension of standard Scheme `case': Each CLAUSE of a
     `qase' statement must have as first element a list containing
     elements which are:

        * literal datums, or

        * a comma followed by the name of a symbolic constant, or

        * a comma followed by an at-sign (@) followed by the name of a
          symbolic constant whose value is a list.

     A `qase' statement is equivalent to a `case' statement in which
     these symbolic constants preceded by commas have been replaced by
     the values of the constants, and all symbolic constants preceded by
     comma-at-signs have been replaced by the elements of the list
     values of the constants.  This use of comma, (or, equivalently,
     `unquote') is similar to that of `quasiquote' except that the
     unquoted expressions must be "symbolic constants".

     Symbolic constants are defined using `defconst', their values are
     substituted in the head of each `qase' clause during macro
     expansion.  `defconst' constants should be defined before use.
     `qase' can be substituted for any correct use of `case'.

     (defconst unit '1)
     (defconst semivowels '(w y))
     (qase (* 2 3)
       ((2 3 5 7) 'prime)
       ((,unit 4 6 8 9) 'composite))        ==>  composite
     (qase (car '(c d))
       ((a) 'a)
       ((b) 'b))                            ==>  _unspecified_
     (qase (car '(c d))
       ((a e i o u) 'vowel)
       ((, at semivowels) 'semivowel)
       (else 'consonant))                   ==>  consonant