Proposed ASDF patch [1/2] - :weakly-depends-on

Adds :weakly-depends-on, which allows a system
to depend upon another system if that system is present in the system
and to ignore the dependency if it is not.

This is nice for optional-functionality.

There was talk on #lisp (yeah, there's ALWAYS talk on #lisp) about
more sophisticated methods that relied upon predicates, that allowed
options to be passed to the loader, etc, etc, but this was quick, easy,
and instantly useful.

I hope you'll find it worthy of inclusion,
Alan

--- /home/shieldsa/code/asdf-cvs/asdf.lisp	2005-09-26 17:54:42.000000000 -0500
+++ asdf.lisp	2005-11-17 17:10:57.000000000 -0600
@@ -930,10 +930,11 @@
 	      ;; remove-keys form.  important to keep them in sync
 	      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 depends-on components in-order-to)
+    (check-component-input type name weakly-depends-on depends-on components in-order-to)
 
     (when (and parent
 	     (find-component parent name)
@@ -946,14 +947,17 @@
     (let* ((other-args (remove-keys
 			'(components pathname default-component-class
 			  perform explain output-files operation-done-p
+			  weakly-depends-on
 			  depends-on serial in-order-to)
 			rest))
 	   (ret
 	    (or (find-component parent name)
 		(make-instance (class-for-type parent type)))))
+      (when weakly-depends-on
+	(setf depends-on (append depends-on (remove-if (complement #'find-system) weakly-depends-on))))
       (when (boundp '*serial-depends-on*)
 	(setf depends-on
-	      (concatenate 'list *serial-depends-on* depends-on)))
+	      (concatenate 'list *serial-depends-on* depends-on)))      
       (apply #'reinitialize-instance
 	     ret
 	     :name (coerce-name name)
@@ -1009,11 +1013,15 @@
 		  (component-inline-methods ret))))
       ret)))
 
-(defun check-component-input (type name depends-on components in-order-to)
+(defun check-component-input (type name weakly-depends-on depends-on components in-order-to)
   "A partial test of the values of a component."
+  (when weakly-depends-on (warn "We got one! XXXXX"))
   (unless (listp depends-on)
     (sysdef-error-component ":depends-on must be a list."
 			    type name depends-on))
+  (unless (listp weakly-depends-on)
+    (sysdef-error-component ":weakly-depends-on must be a list."
+			    type name weakly-depends-on))
   (unless (listp components)
     (sysdef-error-component ":components must be NIL or a list of components."
 			    type name components))


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