PROG1 as a function

[Guy Steele <gls@Think.COM>]

   Date: Wed, 4 Jan 89 14:17 EST
   From: Michael Greenwald <Greenwald@stony-brook.scrc.symbolics.com>

       Date: Wed, 4 Jan 89 13:49 EST
       From: David A. Moon <Moon@STONY-BROOK.SCRC.Symbolics.COM>

	   Date: Tue, 3 Jan 89 20:10 EST
	   From: Barry Margolin <barmar@Think.COM>

	   One of our users noticed today that Symbolics defines PROG1 as a
	   function, rather than as a macro as CLtL specifies.  The definition is
	   essentially

		   (defun prog1 (first-form &rest rest-forms)
		     (declare (ignore rest-forms))
		     first-form)

	   This works because CL requires left-to-right evaluation of function
	   arguments

       CLtL is vague about the order of evaluation of function arguments.  In
       chapters 5 and 7 it neither says that the order is left-to-right nor
       that the order is undefined, and I suspect that the person who made
       PROG1 a macro thought the order was undefined; I can't see any other
       reason that it would need to be a macro.  The place in CLtL that says
       that all function arguments are evaluated left-to-right is page 194,
       buried in the middle of a discussion on numeric contagion.

   Actually, CLtL pg 61 says that the arguments and parameters are
   processed in order, from left to right.  I don't know if "processed"
   implies "evaluated", but I always assumed (perhaps incorrectly) it did.

I interpret this as referring to how the (fully evaluated) arguments
are processed during lambda-binding, not to the order in which argument
forms in a function call are evaluated.  After all, the arguments referred
to on page 61 might have come from a list given to APPLY, rather then
from EVAL on a function call.

--Guy