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)
	      03-Apr-89, Version 4 by Loosemore (amendments)
Status:	      Proposal NEW-MACRO passed at March 89 meeting.

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 function COMPILE-FILE should
  provide the effect of (WITH-COMPILATION-UNIT (:OVERRIDE NIL) ...)
  around its 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.