Cleanup Issue MACRO-DECLARATIONS

Forum
Cleanup
Category
CLARIFICATION
References
Issue SYMBOL-MACROLET-DECLARE Issue DECLARATION-SCOPE Issue DECLARE-TYPE-FREE Issue DEFINE-COMPILER-MACRO Issue DYNAMIC-EXTENT Issue DYNAMIC-EXTENT-FUNCTION Declarations (CLtL chapter 9)

Problem Description

Issue SYMBOL-MACROLET-DECLARE defines a meaning for TYPE declarations when the lexically visible "binding" of the symbol names a symbol-macro. It also requires SYMBOL-MACRO to signal an error if a SPECIAL declaration is provided in the body for a symbol which it defines as a symbol-macro.

What is the meaning of a SPECIAL declaration appearing in some other context (such as in a LOCALLY construct), for a symbol for which the lexically apparent "binding" is a symbol-macro definition? How about any other declaration which normally applies to a variable or function binding (IGNORE, DYNAMIC-EXTENT, FTYPE, INLINE, NOTINLINE) when the lexically apparent "binding" is a macro or symbol-macro definition?

Proposal (MAKE-EXPLICIT)

Clarify that the standard declarations that apply to function or variable bindings have the following effects when the binding is a macro or symbol-macro:

SPECIAL SYMBOL-MACROLET signals an error if it includes a SPECIAL declaration for any symbol that it binds as a symbol-macro. [Issue SYMBOL-MACROLET-DECLARE] Presumably, this error is of type PROGRAM-ERROR and is signalled at compile-time rather than run-time.

A SPECIAL declaration for a symbol whose lexically visible binding is a symbol-macro causes that binding to be shadowed, in the same way that a SPECIAL declaration shadows lexical variable bindings.

TYPE A TYPE declaration for a symbol that names a symbol-macro is equivalent to wrapping a THE expression around the expansion of that symbol-macro. [Issue SYMBOL-MACROLET-DECLARE] This is meaningful regardless of whether the declaration appears in the form that bound the symbol-macro or as a free declaration within the scope of the symbol-macro binding. Multiple TYPE declarations applying to a single symbol-macro binding are handled in the same way as multiple TYPE declarations that apply to a single variable binding.

FTYPE This declaration is not valid when the lexically apparent binding is a macro binding rather than a function binding. (This is because FTYPE declares the functional binding of the name to be of a particular subtype of FUNCTION, and macros are not FUNCTIONs.)

IGNORE IGNORE declarations for symbol-macro bindings should be treated in exactly the same way as IGNORE declarations for variable bindings. In other words, such a declaration specifies that the bindings of of the specified symbol-macros are never used.

DYNAMIC-EXTENT This declaration is not valid when the lexically apparent binding is a symbol-macro or macro binding rather than a variable or function binding.

NOTINLINE In the presence of compiler-macro definitions, this declaration affects references to macros in exactly the same way that it affects references to functions. When the lexically apparent binding is a macro that also has a compiler-macro definition, this declaration can be used to indicate to a language processor that the macro (and not the compiler-macro) definition should be used. [Issue DEFINE-COMPILER-MACRO.] A NOTINLINE declaration for a macro has otherwise has no effect on its expansion. Implementations are not free to ignore this declaration.

INLINE To parallel treatment of NOTINLINE, in the presence of compiler-macro definitions, this declaration affects references to macros in exactly the same way that it affects references to functions. When the lexically apparent binding is a macro that also has a compiler-macro definition, this declaration can be used to indicate to a language processor that the compiler-macro (and not the macro) definition should be used. An INLINE declaration for a macro otherwise has no effect on its expansion. Implementations are free to ignore this declaration.

In those situations where the use of the declaration is not valid, the consequences of evaluating or compiling the program are undefined.

Rationale

This proposal is primarily an explicit restatement of things which have already been stated in other places, with some obvious interpolations added.

Leaving the consequences undefined permits implementations to signal an error, to assign some implementation-specific interpretation to the declaration, or simply to ignore the declaration.

Current Practice

Utah Common Lisp implements this proposal. It currently ignores all declarations that apply to function or variable bindings when the lexically visible binding is a macro or symbol-macro. The declarations are added to the environment in the normal way but are never examined by the interpreter or compiler. The exception is that a SPECIAL declaration will shadow a symbol-macro definition in the same way that it will shadow a lexical variable binding.

Neither HPCL-I nor Lucid CL complain about FTYPE or INLINE/NOTINLINE declarations when the lexically visible function "binding" is a macro. They are apparently being ignored. KCL ignores all declarations that apply to function bindings (and doesn't yet support symbol-macros).

Cost to Implementors

Presumably small.

Cost to Users

It seems unlikely that this proposal would be an incompatible change that causes many user programs to break, particularly given the relative newness of symbol-macros and compiler-macros.

Benefits

More complete specification of the language and less chance for confusion to arise later on.

Aesthetics

Some people might be bothered by the asymmetry between the handling of TYPE and FTYPE declarations. Strictly speaking, the special handling for TYPE declarations is unnecessary since one could explicitly include the THE form in the expansion of the symbol-macro. Other people probably think that the notational convenience outweighs the asymmetry.

Discussion

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