Re: Some easy ones (?)

[NGALL@G.BBN.COM]

    Date: Wed, 23 Jul 86 13:08 EDT
    From: David C. Plummer <DCP@QUABBIN.SCRC.Symbolics.COM>
    To: NGALL@G.BBN.COM, Fahlman@C.CS.CMU.EDU
    Subject: Re: Some easy ones (?)
    In-Reply-To: <[G.BBN.COM]23-Jul-86 02:30:50.NGALL>
    Message-ID: <860723130828.1.DCP@FIREBIRD.SCRC.Symbolics.COM>
    
	Date: 23 Jul 1986 02:30-EDT
	From: NGALL@G.BBN.COM
    
	Taking into account the other comments on this function (the ones that
	I agree with at least), how about this proposal
	(PARSE-BODY body &optional environment
				   declarations-allowed-p
				   doc-string-allowed-p)
	Parses a BODY argument of the form
	({declaration|doc-string}* {form}*)
	    ...
    
    Until somebody says something profound to change my mind, I hold that if
    a "body" may not contain either declarations or doc-strings, it is not a
    true body, and therefore you have no right to call PARSE-BODY.  I have
    no idea what the reference to CASE and PROGN was.  If the claim that
    each clause of a CASE contained a "body" you are wrong; it contains an
    "implicit PROGN".  Implicit PROGNs are not suitable arguments to
    PARSE-BODY.
    
This may not be profound, but its true.  A robust macro definition of
CASE, i.e., one that performs an expansion time check to ensure that
the initial forms after the KEYFORM are not declarations, can be
written much more easily with DECLARATIONS-ALLOWED-P, e.g.,

(defmacro case (keyform &rest clauses &environment env)
  (multiple-value-bind (decls doc clauses)
                       (parse-body clauses env nil)
                       (declare (ignore decls doc))
    ;; At this point I can rest assured that parse-body has detected
    ;; any illegal declarations.
    ...)

Things are easier still if we adopt my proposal for &body:

(defmacro case (keyform &body (&forms clauses))
  ;; At this point I can rest assured that parse-body has detected
  ;; any illegal declarations.
  ...)

Profound enough?

-- Nick