Forum: Compiler Issue: COMPILE-FILE-HANDLING-OF-TOP-LEVEL-FORMS References: CLtL pages 66-70, 143 Category: CLARIFICATION Edit history: V1, 07 Oct 1987 Sandra Loosemore V2, 15 Oct 1987 Sandra Loosemore V3, 15 Jan 1988 Sandra Loosemore V4, 06 May 1988 Sandra Loosemore V5, 20 May 1988 Sandra Loosemore V6, 09 Jun 1988 Sandra Loosemore V7, 16 Dec 1988 Sandra Loosemore (Comments from Pitman, change DEFCONSTANT, etc.) V8, 31 Dec 1988 Sandra Loosemore (CLOS additions, etc.) V9, 23 Jan 1989 Sandra Loosemore (remove the CLOS additions again) Status: Proposal CLARIFY passed Jan 89 Problem Description: Standard programming practices assume that, when calls to defining macros such as DEFMACRO and DEFVAR are processed by COMPILE-FILE, certain side-effects occur that affect how subsequent forms in the file are compiled. However, these side-effects are not mentioned in CLtL, except for a passing mention that macro definitions must be ``seen'' by the compiler before it can compile calls to those macros correctly. In order to write portable programs, users must know exactly which defining macros have compile-time side-effects and what those side-effects are. Inter-file compilation dependencies are distinct from, and not addressed by, this issue. Proposal: COMPILE-FILE-HANDLING-OF-TOP-LEVEL-FORMS:CLARIFY (1) Clarify that defining macros such as DEFMACRO or DEFVAR, appearing within a file being processed by COMPILE-FILE, normally have compile-time side effects which affect how subsequent forms in the same file are compiled. A convenient model for explaining how these side effects happen is that the defining macro expands into one or more EVAL-WHEN forms, and that the calls which cause the compile-time side effects to happen appear in the body of an (EVAL-WHEN (COMPILE) ...) form. (2) The affected defining macros and their specific side effects are as follows. In each case, it is identified what users must do to ensure that their programs are conforming, and what compilers must do in order to correctly process a conforming program. DEFTYPE: Users must ensure that the body of a DEFTYPE form is evaluable at compile time if the type is referenced in subsequent type declarations. The compiler must ensure that the DEFTYPE'd type specifier is recognized in subsequent type declarations. If the expansion of a type specifier is not defined fully at compile time (perhaps because it expands into an unknown type specifier or a SATISFIES of a named function that isn't defined in the compile-time environment), an implementation may ignore any references to this type in declarations and/or signal a warning. DEFMACRO, DEFINE-MODIFY-MACRO: The compiler must store macro definitions at compile time, so that occurrences of the macro later on in the file can be expanded correctly. Users must ensure that the body of the macro is evaluable at compile time if it is referenced within the file being compiled. DEFUN: DEFUN is not required to perform any compile-time side effects. In particular, DEFUN does not make the function definition available at compile time. An implementation may choose to store information about the function for the purposes of compile-time error-checking (such as checking the number of arguments on calls), or to enable the function to be expanded inline. DEFVAR, DEFPARAMETER: The compiler must recognize that the variables named by these forms have been proclaimed special. However, it must not evaluate the initial value form or SETQ the variable at compile time. DEFCONSTANT: The compiler must recognize that the symbol names a constant. An implementation may choose to evaluate the value-form at compile time, load time, or both. Therefore users must ensure that the value-form is evaluable at compile time (regardless of whether or not references to the constant appear in the file) and that it always evaluates to the same value. DEFSETF, DEFINE-SETF-METHOD: The compiler must make SETF methods available so that it may be used to expand calls to SETF later on in the file. Users must ensure that the body of DEFINE-SETF-METHOD and the complex form of DEFSETF are evaluable at compile time if the corresponding place is referred to in a subsequent SETF in the same file. The compiler must make these SETF methods available to compile-time calls to GET-SETF-METHOD when its environment argument is a value received as the &ENVIRONMENT parameter of a macro. DEFSTRUCT: The compiler must make the structure type name recognized as a valid type name in subsequent declarations (as for DEFTYPE) and make the structure slot accessors known to SETF. In addition, the compiler must save enough information about the structure type so that further DEFSTRUCT definitions can :INCLUDE a structure type defined earlier in the file being compiled. The functions which DEFSTRUCT generates are not defined in the compile time environment, although the compiler may save enough information about the functions to code subsequent calls inline. The #S reader syntax may or may not be available at compile time. DEFINE-CONDITION: The rules are essentially the same as those for DEFSTRUCT; the compiler must make the condition type recognizable as a valid type name, and it must be possible to reference the condition type as the parent-type of another condition type in a subsequent DEFINE-CONDITION in the file being compiled. DEFPACKAGE: All of the actions normally performed by this macro at load time must also be performed at compile time. (3) The compile-time side effects may cause information about the definition to be stored differently than if the defining macro had been processed in the "normal" way (either interpretively or by loading the compiled file). In particular, the information stored by the defining macros at compile time may or may not be available to the interpreter (either during or after compilation), or during subsequent calls to COMPILE or COMPILE-FILE. For example, the following code is nonportable because it assumes that the compiler stores the macro definition of FOO where it is available to the interpreter: (defmacro foo (x) `(car ,x)) (eval-when (eval compile load) (print (foo '(a b c)))) A portable way to do the same thing would be to include the macro definition inside the EVAL-WHEN: (eval-when (eval compile load) (defmacro foo (x) `(car ,x)) (print (foo '(a b c)))) Rationale: The proposal generally reflects standard programming practices. The primary purpose of the proposal is to make an explicit statement that CL supports the behavior that most programmers expect and many implementations already provide. The primary point of controversy on this issue has been the treatment of the initial value form by DEFCONSTANT, where there is considerable variance between implementations. The effect of the current wording is to legitimize all of the variants. Current Practice: Many (probably most) Common Lisp implementations, including VaxLisp and Lucid Lisp, are already largely in conformance. In VaxLisp, macro definitions that occur as a side effect of compiling a DEFMACRO form are available to the compiler (even on subsequent calls to COMPILE or COMPILE-FILE), but are not available to the interpreter (even within the file being compiled). By default, Kyoto Common Lisp evaluates *all* top level forms as they are compiled, which is clearly in violation of the behavior specified on p 69-70 of CLtL. There is a flag to disable the compile-time evaluation, but then macros such as DEFMACRO, DEFVAR, etc. do not make their definitions available at compile-time either. Cost to implementors: The intent of the proposal is specifically not to require the compiler to have special knowledge about each of these macros. In implementations whose compilers do not treat these macros as special forms, it should be fairly straightforward to use EVAL-WHENs in their expansions to obtain the desired compile-time side effects. Cost to users: Since CLtL does not specify whether and what compile-time side-effects happen, any user code which relies on them is, strictly speaking, nonportable. In practice, however, most programmers already expect most of the behavior described in this proposal and will not find it to be an incompatible change. Benefits: Adoption of the proposal will provide more definite guidelines on how to write programs that will compile correctly under all CL implementations. Discussion: Reaction to a preliminary version of this proposal on the common-lisp mailing list was overwhelmingly positive. More than one person responded with comments to the effect of "but doesn't CLtL already *say* that somewhere?!?" Others have since expressed a more lukewarm approval. It has been suggested that this proposal should also include PROCLAIM. However, since PROCLAIM is not a macro, its compile-time side effects cannot be handled using the EVAL-WHEN mechanism. A separate proposal seems more appropriate. Item (3) allows for significant deviations between implementations. While there is some sentiment to the effect that the compiler should store definitions in a manner identical to that of the interpreter, other people believe strongly that compiler side-effects should be completely invisible to the interpreter. The author is of the opinion that since this is a controversial issue, further attempts to restrict this behavior should be considered as separate proposals. It should be noted that user-written code-analysis programs must generally treat these defining macros as special forms and perform similar "compile-time" actions in order to correctly process conforming programs.