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<TITLE>CLHS: Issue FUNCTION-CALL-EVALUATION-ORDER Writeup</TITLE>
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<H2>Issue FUNCTION-CALL-EVALUATION-ORDER Writeup</H2>
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<PRE><B>Status:</B> passed v2 on 15-0 vote, Jun-90<P>
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<B>Issue:</B> <A HREF="iss171.htm">FUNCTION-CALL-EVALUATION-ORDER</A><P>
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<P>
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<B>References:</B> CLtL p 194 ("...that order is always left to right")<P>
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ANSI CL spec (Aug 29, 1989 draft) p.4-10 3rd paragraph<P>
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<P>
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<B>Category:</B> CLARIFICATION<P>
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<P>
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<B>Edit history:</B> Version 1 by Clinger (22 March 1988)<P>
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Version 2 by Moon (1 Feb 90), add MORE-UNSPECIFIED<P>
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<P>
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<B>Problem Description:<P>
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</B><P>
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CLtL does not say whether the function expression of a call is evaluated<P>
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before or after the argument expressions.<P>
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<P>
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This is Symbolics issue #4.<P>
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<P>
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<B>Proposal (FUNCTION-CALL-EVALUATION-ORDER:UNSPECIFIED):<P>
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</B><P>
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Common Lisp does not specify whether the function expression of a call is<P>
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evaluated before or after the argument expressions. Programs that rely<P>
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on a particular order of evaluation are in error.<P>
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<P>
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The above proposal was accepted in October 1988. The following proposal<P>
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is new in version 2.<P>
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<P>
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<B>Proposal (FUNCTION-CALL-EVALUATION-ORDER:MORE-UNSPECIFIED):<P>
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</B><P>
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Common Lisp does not specify whether the function expression of a call is<P>
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evaluated before the argument expressions, after the argument<P>
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expressions, or between any two argument expressions if there is more<P>
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than one argument. Programs that rely on a particular order of<P>
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evaluation are in error.<P>
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<P>
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<B>Example:<P>
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</B><P>
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(<A REL=DEFINITION HREF="../Body/m_defun.htm#defun"><B>defun</B></A> foo (x) (+ x 3))<P>
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(<A REL=DEFINITION HREF="../Body/m_defun.htm#defun"><B>defun</B></A> bar () (<A REL=DEFINITION HREF="../Body/a_setf.htm#setf"><B>setf</B></A> (<A REL=DEFINITION HREF="../Body/f_symb_1.htm#symbol-function"><B>symbol-function</B></A> 'foo) #'(<A REL=DEFINITION HREF="../Body/a_lambda.htm#lambda"><B>lambda</B></A> (x) (+ x 4))))<P>
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(foo (<A REL=DEFINITION HREF="../Body/s_progn.htm#progn"><B>progn</B></A> (bar) 20))<P>
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; may return 23 or 24<P>
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<P>
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(<A REL=DEFINITION HREF="../Body/m_defun.htm#defun"><B>defun</B></A> foo2 (x y) (+ x y))<P>
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(<A REL=DEFINITION HREF="../Body/m_defun.htm#defun"><B>defun</B></A> bar2 () (<A REL=DEFINITION HREF="../Body/a_setf.htm#setf"><B>setf</B></A> (<A REL=DEFINITION HREF="../Body/f_symb_1.htm#symbol-function"><B>symbol-function</B></A> 'foo2) #'(<A REL=DEFINITION HREF="../Body/a_lambda.htm#lambda"><B>lambda</B></A> (x y) (* x y))))<P>
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(<A REL=DEFINITION HREF="../Body/m_defun.htm#defun"><B>defun</B></A> bar3 () (<A REL=DEFINITION HREF="../Body/a_setf.htm#setf"><B>setf</B></A> (<A REL=DEFINITION HREF="../Body/f_symb_1.htm#symbol-function"><B>symbol-function</B></A> 'foo2) #'(<A REL=DEFINITION HREF="../Body/a_lambda.htm#lambda"><B>lambda</B></A> (x y) (- x y))))<P>
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(foo2 (<A REL=DEFINITION HREF="../Body/s_progn.htm#progn"><B>progn</B></A> (bar2) 6) (<A REL=DEFINITION HREF="../Body/s_progn.htm#progn"><B>progn</B></A> (bar3) 7))<P>
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; under UNSPECIFIED, may return 13 or -1<P>
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; under MORE-UNSPECIFIED, may return 13, -1, or 42<P>
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<P>
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<B>Rationale:<P>
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</B><P>
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UNSPECIFIED makes the status quo explicit.<P>
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<P>
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MORE-UNSPECIFIED allows complete freedom to the implementation; as long<P>
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as we are not going to <A REL=DEFINITION HREF="../Body/f_provid.htm#require"><B>require</B></A> all implementations to be consistent, it<P>
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seems useless to impose half a restriction on the implementation. Some<P>
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RISC machines with delayed branches would prefer to evaluate the function<P>
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between evaluating the penultimate argument and the last argument in<P>
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some situations.<P>
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<P>
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<B>Current Practice:<P>
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</B> <P>
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TI and Symbolics used to evaluate the function expression last.<P>
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Symbolics currently evaluates the function expression either first or<P>
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last depending on the hardware and whether the code is compiled or<P>
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interpreted. I'm not sure if TI has changed in the past two years.<P>
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Lucid and Coral sometimes evaluate the function expression first and at<P>
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other times evaluate the function expression last.<P>
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<P>
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<B>Cost to implementors:<P>
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</B><P>
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None.<P>
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<P>
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<B>Cost to users:<P>
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</B><P>
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None.<P>
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<P>
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<B>Benefits:<P>
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</B><P>
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Codifies an ambiguity.<P>
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<P>
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<B>Aesthetics:<P>
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</B> <P>
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Since Common Lisp evaluates argument expressions from left to right, it<P>
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would be more consistent for the function expression to be evaluated<P>
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before the argument expressions. On the other hand, the evaluation rules<P>
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for function expressions are already quite different from the rules for<P>
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argument expressions, and nobody in their right mind would write code<P>
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that depends on the order of evaluation, so aesthetics should not count<P>
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for much in this case. Requiring left to right evaluation would force<P>
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some implementations to consume an extra register for many function<P>
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calls. The efficiency argument seems more important than the aesthetic<P>
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argument.<P>
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</PRE>
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