Suggested enhancement: a :do-first keyword for defining systems

I would like to suggest a slight enhancement to asdf: the ability to
specify :do-first dependencies in system declarations.  The change has
both practical and pedagogical benefits.

Here is a practical application.  Suppose I have a pre-processor that
takes a file named, e.g., "foo.xyz" and produces "foo.lisp" from it.
But it's not just a simple pre-processor.  It's an extensible
pre-processor that allows you to customize its behavior by
writing Lisp code.

So I have a bunch of ".xyz" files, and a file called "aux.lisp" that
contains auxiliary functions that the preprocessor uses for my
particular ".xyz" files.  I define a new component class called
xyz-source-file, based on source-file, and a new operation class
called preproc-op whose perform method runs the preprocessor.  I want
to define a system that can turn the ".xyz" files into ".lisp" files.
To do that, they need for "aux.fasl" to be loaded.

I use an ordinary :in-order-to dependency for each of the generated
".lisp" files to say that it is dependent on its corresponding ".xyz"
file.

I also want to say that these transformations are dependent on
"aux.fasl" being loaded.

If all of the ".lisp" files already exist, and each of them has a
creation date greater than that of their corresponding ".xyz" file,
then there is no work to be done, and I do not want "aux.fasl" to be
loaded, since it is not needed.

To obtain this effect, I need to express the dependence on "aux.fasl"
using a :do-first dependency rather than an :in-order-to dependency.
If I use :in-order-to, "aux.fasl" will be loaded unnecessarily.

There is also a pedagogical reason.  Talking to my co-workers, I find
that there is some confusion about exactly what :depends-on means.  In
reality, down inside asdf, the decision making is really driven by the
two kinds of dependencies (:in-order-to and :do-first).  Once you
understand what these two are -- and I think you need to understand
them in order to do what I suggested above -- then you can explain
:depends-on very precisely by expressing it in terms of :in-order-to
and :do-first.  :depends-on is essentially a kind of specialized
"macro" that "expands" into the two kinds of dependency.  By
explaining :depends-on in this way, you can completely explain its
behavior.

Here is the suggested change:

*** asdf.lisp	Thu Sep 21 22:23:01 2006
--- do-first-asdf.lisp	Thu Oct  5 01:08:46 2006
***************
*** 1004,1010 ****
                components pathname default-component-class
                perform explain output-files operation-done-p
                weakly-depends-on
!               depends-on serial in-order-to
                ;; list ends
                &allow-other-keys) options
      (check-component-input type name weakly-depends-on
                             depends-on components in-order-to)
--- 1004,1010 ----
                components pathname default-component-class
                perform explain output-files operation-done-p
                weakly-depends-on
!               depends-on serial in-order-to do-first
                ;; list ends
                &allow-other-keys) options
      (check-component-input type name weakly-depends-on 
                             depends-on components in-order-to)
***************
*** 1067,1073 ****
               in-order-to
               `((compile-op (compile-op ,@depends-on))
                 (load-op (load-op ,@depends-on))))
!             (slot-value ret 'do-first) `((compile-op (load-op ,@depends-on))))
  
        (loop for (n v) in `((perform ,perform) (explain ,explain)
                             (output-files ,output-files)
--- 1067,1076 ----
               in-order-to
               `((compile-op (compile-op ,@depends-on))
                 (load-op (load-op ,@depends-on))))
!             (slot-value ret 'do-first)
! 	    (union-of-dependencies
! 	     do-first
! 	     `((compile-op (load-op ,@depends-on)))))
  
        (loop for (n v) in `((perform ,perform) (explain ,explain)
                             (output-files ,output-files)

-- Dan Weinreb



-------------------------------------------------------------------------
Take Surveys. Earn Cash. Influence the Future of IT
Join SourceForge.net's Techsay panel and you'll get the chance to share your
opinions on IT & business topics through brief surveys -- and earn cash
http://www.techsay.com/default.php?page=join.php&p=sourceforge&CID=DEVDEV