issue COMPILE-FILE-HANDLING-OF-TOP-LEVEL-FORMS, v9

[sandra%defun@cs.utah.edu (Sandra J Loosemore)]

This is the version that was passed at the Hawaii meeting.

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 Sanrda Loosemore
			(remove the CLOS additions again)


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.

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