src/: Introduce a macro for defining on-demand slots.

[sod] / src / utilities.lisp

diff --git a/src/utilities.lisp b/src/utilities.lisp
index 5c061bb..98d314a 100644 (file)
--- a/src/utilities.lisp
+++ b/src/utilities.lisp
@@ -313,6 +313,9 @@
    except where overridden by INITARGS."
   (apply #'copy-instance-using-class (class-of object) object initargs))
 
+(export '(generic-function-methods method-specializers
+         eql-specializer eql-specializer-object))
+
 ;;;--------------------------------------------------------------------------
 ;;; List utilities.
 
@@ -378,7 +381,7 @@
    the input LISTS in the sense that if A precedes B in some input list then
    A will also precede B in the output list.  If the lists aren't consistent
    (e.g., some list contains A followed by B, and another contains B followed
-   by A) then an error of type INCONSISTENT-MERGE-ERROR is signalled.
+   by A) then an error of type `inconsistent-merge-error' is signalled.
 
    Item equality is determined by TEST.
 
@@ -618,7 +621,7 @@
 (defun symbolicate (&rest symbols)
   "Return a symbol named after the concatenation of the names of the SYMBOLS.
 
-   The symbol is interned in the current *PACKAGE*.  Trad."
+   The symbol is interned in the current `*package*'.  Trad."
   (intern (apply #'concatenate 'string (mapcar #'symbol-name symbols))))
 
 ;;;--------------------------------------------------------------------------
@@ -629,7 +632,7 @@
     ((object stream &rest args) &body body)
   "Print helper for usually-unreadable objects.
 
-   If *PRINT-ESCAPE* is set then print OBJECT unreadably using BODY.
+   If `*print-escape*' is set then print OBJECT unreadably using BODY.
    Otherwise just print using BODY."
   (with-gensyms (print)
     `(flet ((,print () ,@body))
@@ -691,6 +694,38 @@
                      ,(loopguts indexvar t nil))))))))))
 
 ;;;--------------------------------------------------------------------------
+;;; Structure accessor hacks.
+
+(export 'define-access-wrapper)
+(defmacro define-access-wrapper (from to &key read-only)
+  "Make (FROM THING) work like (TO THING).
+
+   If not READ-ONLY, then also make (setf (FROM THING) VALUE) work like
+   (setf (TO THING) VALUE).
+
+   This is mostly useful for structure slot accessors where the slot has to
+   be given an unpleasant name to avoid it being an external symbol."
+  `(progn
+     (declaim (inline ,from ,@(and (not read-only) `((setf ,from)))))
+     (defun ,from (object)
+       (,to object))
+     ,@(and (not read-only)
+           `((defun (setf ,from) (value object)
+               (setf (,to object) value))))))
+
+(export 'define-on-demand-slot)
+(defmacro define-on-demand-slot (class slot (instance) &body body)
+  "Defines a slot which computes its initial value on demand.
+
+   Sets up the named SLOT of CLASS to establish its value as the implicit
+   progn BODY, by defining an appropriate method on `slot-unbound'."
+  (with-gensyms (classvar slotvar)
+    `(defmethod slot-unbound
+        (,classvar (,instance ,class) (,slotvar (eql ',slot)))
+       (declare (ignore ,classvar))
+       (setf (slot-value ,instance ',slot) (progn ,@body)))))
+
+;;;--------------------------------------------------------------------------
 ;;; CLOS hacking.
 
 (export 'default-slot)