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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.


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