A plurality of defsystems [was Re: rfc - delay the parsing of system components ]

Gary King writes:

> * I don't want to replace ASDF; it;s good, it works, it's in use;
> adding a new one just muddies the waters.

For my part, I don't see any reason why the Lisp community should
constrain itself to having exactly one defsystem facility.  ASDF is
useful, but it has shortcomings, and since it's never clear which of
ASDF's details are intended or accident, and in any case which details
are important to anybody, ISTM that it would probably be a good thing if
alternative defsystems could co-exist.

So I wonder how hard it would be to introduce an interface that
insulated users and programs from the details of how a system's
construction and loading happens to be implemented.  Something like the
following seems as though it would suffice:

--
BUILD-SYSTEM <NAME> &key &allow-other-keys                   [Function]

Call successive functions in *SYSTEM-BUILDER-HOOKS* using <NAME>, the
list of keywords, and :ALLOW-OTHER-KEYS T, until one returns true, and
signals an error if no function in *SYSTEM-BUILDER-HOOKS* returns true.

*SYSTEM-BUILDER-HOOKS*                                       [Variable]

A list of designators for functions that take a string designator and a
list of keywords.  When called, each function will attempt to build a
system identified by the string designator and possibly some of the
keyword arguments, and return true if such a system been built, or NIL
otherwise.  Building a system might have arbitrary side-effects on the
Lisp instance, but may not cause it to terminate.

LOAD-SYSTEM <NAME> &key &allow-other-keys                    [Function]

Call successive functions in *SYSTEM-LOADER-HOOKS* using <NAME>, the
list of keywords, and :ALLOW-OTHER-KEYS T until one returns true, and
signals an error if no function in *SYSTEM-LOADER-HOOKS* returns true.

*SYSTEM-LOADER-HOOKS*                                        [Function]

A list of designators for functions that take a string designator and a
list of keywords.  When called, each function will attempt to load a
system identified by the string designator and possibly some of the
keyword arguments.
--

So, for example, ASDF could install a hook for each of the two variables
that called OPERATE with suitable arguments, and other defsystems could
do analogous things.  Library management tools like asdf-install could
indirect through BUILD-SYSTEM and LOAD-SYSTEM.  Inter-system
dependencies could translate into to calls to LOAD-SYSTEM (possibly
after trying a defsystem-specific mechanism like what ASDF does now).
Since both of these functions takes keywords, things like system
versions or arguments to the construction or loading machinery can be
supplied and their interpretation is left to an underlying defsystem
tool.

Thoughts?

--
Richard

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