#

## Issue MACROEXPAND-RETURN-VALUE Writeup

    Forum:		Cleanup
    Issue:		MACROEXPAND-RETURN-VALUE
    References:	MACROEXPAND, MACROEXPAND-1
    			(CLtL p151)
    		COMPILER-MACROEXPAND, COMPILER-MACROEXPAND-1
    			(issue DEFINE-COMPILER-MACRO)
    Category:	CHANGE
    Edit History:   V1, 29 Oct 1989, Sandra Loosemore

    Problem Description:

    CLtL says that the second value returned by MACROEXPAND and
    MACROEXPAND-1 is T if the argument form is a macro call.  Presumably
    this is also the case for COMPILER-MACROEXPAND and
    COMPILER-MACROEXPAND-1, since these functions are intended to be "just
    like" their counterparts.  Specifying a return value of T is
    inconsistent with other predicates in the language, where a return
    value of "true" is normally specified instead.

    Proposal (MACROEXPAND-RETURN-VALUE:TRUE):

    Change the specification so that the second return value from
    MACROEXPAND, MACROEXPAND-1, COMPILER-MACROEXPAND, and
    COMPILER-MACROEXPAND-1 is a boolean instead of one of the symbols T or
    NIL.  In other words, if the form represents a macro call (or a
    compiler macro call, as appropriate), the second return value is true.

    Rationale:

    This is more consistent with other predicates in the language.

    Current Practice:

    Presumably all conforming implementations now return T.

    Cost to implementors:

    None, except possibly for the cost of changing documentation.

    Cost to users:

    It seems unlikely that user programs would explicitly test for the
    return value being T, or otherwise depend on this.

    Benefits:

    Increased consistency in the language design.

    Discussion:

    Loosemore notes:
      In some implementations, there is a slight performance advantage in
      allowing the return values from predicates to be any non-NIL value
      instead of the symbol T.  Some functions such as MEMBER are required
      to return a particular truth value that conveys extra information back
      to the caller, but there doesn't appear to be similar motivation for
      making an exception to the general rule in this situation.

Copyright 1996-2005, LispWorks Ltd. All rights reserved.