Issue COMPILE-FILE-HANDLING-OF-TOP-LEVEL-FORMS Writeup

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.