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Edit System is anti-modularity
Date: Wed, 8 May 1991 09:30 EDT
From: Tom Shepard <Shepard@MCKENZIE.S&C.Dialnet.Symbolics.COM>
From: Dodds@YUKON.SCRC.Symbolics.COM (Douglas Dodds)
I would also comment that making (for example) IP-TCP a component system
of your own application's system is not the right way to require the
presence of a generally used system that your system depends on. The
right construct for that is to use :REQUIRED-SYSTEMS (:IP-TCP) as an
option in your DEFSYSTEM.
This is not documented [as of 8.0.1]. How is the behavior of
:REQUIRED-SYSTEMS different from using component systems?
Sorry, I thought it was, but wasn't sure. (In the examples that follow,
the systems that go with the keyword can either be a single system name
or symbol, or a list of them.)
:required-systems Option for defsystem
Specifies the systems that are required to be loaded before this sys-
tem being defined can be loaded (or compiled). If you try to load a
system when a required system is missing, SCT gives an error telling
you that the required system must be loaded first.
:required-systems allows you to establish dependencies among
several systems without having to lock in a specific version number,
as would happen if one system were a component system of the oth-
(:pretty-name "Automatic Registration System"
:required-systems "Scheduling Utilities")...)
(See also :LOAD-WHEN-SYSTEMS-LOADED, for optional parts of your system
that have dependencies on generally used systems that might or might not
No doubt this is also first documented in 8.1, so to make things clear:
:load-when-systems-loaded Option for :module
The :load-when-systems-loaded option instructs SCT not to load
some modules of a system when a set of required systems is not
loaded. When all the required systems become loaded, SCT automati-
cally loads the unloaded modules.
Including this option in a module has two effects:
o If the required systems are not all loaded, that module is not
o When all the required systems become loaded, SCT goes back and
loads the module.
o You cannot safely patch that module, as it might not be loaded yet
at the time patches are loaded.
:load-when-systems-loaded differs from the :required-systems
option to defsystem in that :required-systems gives an error if
the required systems are not present. :load-when-systems-loaded
never gives an error.
Note that any module that contains the :load-when-systems-
loaded option should be a named module, so SCT can keep track of
the unloaded modules by name (since sysdcls can be reloaded).
(:module unix-spooler ("ux-spool")
(:serial (:parallel "defs" "macros")
Suppose that the system UNIX-SUPPORT is not loaded. When you load
the PRINT-SPOOLER system, all the files in the system are loaded ex-
cept for those files in UNIX-SPOOLER module (namely, UX-SPOOL). If
and when you load the UNIX-SUPPORT system, the files in the UNIX-
SPOOLER will get loaded.