Issue WITH-COMPILATION-UNIT Writeup

Forum:	      Compiler

Issue:        WITH-COMPILATION-UNIT

References:   COMPILE (p438), COMPILE-FILE (p439)

Category:     ADDITION

Edit history: 29-Sep-88, Version 1 by Pitman

	      10-Mar-89, Version 2 by Pitman (merge comments)

	      13-Mar-89, Version 3 by Loosemore (update discussion)

	      10-Apr-89, Version 4 by Pitman (wording change per X3J13)

Status:	      Ready for release


Problem Description:


  Some actions done by the compiler (and particularly the file compiler)

  are typically deferred until the "very end" of compilation.  For example,

  some compilers complain about "functions seen but not defined".


  Unfortunately, since COMPILE-FILE is the largest unit of granularity,

  and since systems often extend over more than one file, it often happens

  that the compiler needlessly complains at the end of compiling one file

  about the absence of functions defined in the next file. 


Proposal (WITH-COMPILATION-UNIT:NEW-MACRO):


  Add the following new macro:


   WITH-COMPILATION-UNIT options &BODY forms			[Macro]


   Executes forms from left to right. Within the dynamic context

   of this form, warnings deferred by the compiler until "the end of

   compilation" will be deferred until the end of the outermost call

   to WITH-COMPILATION-UNIT. The result(s) are the same as that of

   the last of the FORMS (or NIL if FORMS is null).


   OPTIONS is a keyword/value list, where only the values are

   evaluated. The set of keywords permitted may be extended by the

   implementation, but the only keyword defined by this standard is:


     :OVERRIDE boolean


       The default is NIL. If nested dynamically only the outer call

       to WITH-COMPILATION-UNIT has any effect unless BOOLEAN is T,

       in which case warnings are deferred only to the end of the

       innermost call.


  It follows that the functions COMPILE and COMPILE-FILE should

  provide the effect of (WITH-COMPILATION-UNIT (:OVERRIDE NIL) ...)

  around their code.


  Any implementation-dependent extensions may only be provided

  as the result of an explicit programmer request by use of 

  an implementation-dependent keyword.  Implementations are forbidden

  from attaching additional meaning to a conforming use of this

  macro.


  Note also that not all warnings are deferred. In some implementations,

  it may be that none are deferred. This proposal only creates an

  interface to the capability where it exists, it does not require the

  creation of the capability. An implementation which does not do 

  deferred warnings may correctly implement this as expanding into PROGN.


Test Case:


  (DEFUN COMPILE-FILES (&REST FILES)

    (WITH-COMPILATION-UNIT ()

      (MAPCAR #'(LAMBDA (FILE) (COMPILE-FILE FILE)) FILES)))


  (COMPILE-FILES "A" "B" "C")


  processes deferred warnings only after compiling all of A, B, and C.


Rationale:


  This will make the development of portable system-construction tools

  considerably more convenient. 


Current Practice:


  Lucid has a very similar facility, called WITH-DEFERRED-WARNINGS.


  TI Explorer and Symbolics Genera have a similar facility, which they

  call COMPILER-WARNING-CONTEXT-BIND.


Cost to Implementors:


  In implementations which have no deferred warnings, there is no cost.

  

  In implementations which have deferred warnings, the cost is probably

  fairly small -- usually just a matter of writing interfacing the 

  proposed macro to an existing one.


Cost to Users:


  None. This is a compatible addition.


Cost of Non-Adoption:


  Portable system-construction tools would continue to print lots of

  spurious warnings because they would have no way to tell the system

  that a set of files was working together.


Benefits:


  The cost of non-adoption is avoided.


Aesthetics:


  The ability to create a compilation unit other than a file is important.


Discussion:


  Pitman and Benson support this addition.


  One could imagine adding more options at a later date.


  It was the opinion of the compiler committee that there was room for

  expansion here to address issues like bounding the scope of global

  proclamations, sharing compile-time environments across files, etc.

  However, insufficient work was done on this to justify putting such

  a thing into the standard. The only clear need we have at this time

  was to defer warnings, but we chose a general name like 

  WITH-COMPILATION-UNIT rather than a specific name like Lucid's

  WITH-DEFERRED-WARNINGS in order to encourage implementations to 

  experiment with other kinds of options under implementation-specific

  keywords. Perhaps by the time of the next standard there will be

  sufficient understanding of this area to warrant further elaboration

  of this primitive.


  Kim Barrett says:


    I strongly oppose the behavior you proposed for compile and

    compile-file.  It is my belief that whether to override or not must be

    controlled through an argument to the compile functions, with the

    default being to override.  Otherwise, all existing code which makes

    use of the compile functions must be modified to protect itself by

    wrapping a (with-compilation-unit (:override t) ...) around the calls

    to the compiler.

    

    Consider a stream system built on an object system which will compose

    and compile functions on the fly on an as needed basis.  It would be

    very strange for the functions so generated while doing file io for

    the user's compile-file to have any relationship with said

    compile-file.

    

    I agree with your position that implementation-dependent extensions

    must be explicitly requested.