X-Git-Url: https://git.distorted.org.uk/~mdw/sod/blobdiff_plain/141283ff3851f6ada772db21a496026fd8fdd76e..6ac5b807942124461e19a7964165d7237c61fbb7:/src/utilities.lisp

diff --git a/src/utilities.lisp b/src/utilities.lisp
index 1093f68..d1755da 100644
--- a/src/utilities.lisp
+++ b/src/utilities.lisp
@@ -195,6 +195,66 @@
 	    body)))
 
 ;;;--------------------------------------------------------------------------
+;;; Locatives.
+
+(export '(loc locp))
+(defstruct (loc (:predicate locp) (:constructor make-loc (reader writer)))
+  "Locative data type.  See `locf' and `ref'."
+  (reader nil :type function)
+  (writer nil :type function))
+
+(export 'locf)
+(defmacro locf (place &environment env)
+  "Slightly cheesy locatives.
+
+   (locf PLACE) returns an object which, using the `ref' function, can be
+   used to read or set the value of PLACE.  It's cheesy because it uses
+   closures rather than actually taking the address of something.  Also,
+   unlike Zetalisp, we don't overload `car' to do our dirty work."
+  (multiple-value-bind
+      (valtmps valforms newtmps setform getform)
+      (get-setf-expansion place env)
+    `(let* (,@(mapcar #'list valtmps valforms))
+       (make-loc (lambda () ,getform)
+		 (lambda (,@newtmps) ,setform)))))
+
+(export 'ref)
+(declaim (inline ref (setf ref)))
+(defun ref (loc)
+  "Fetch the value referred to by a locative."
+  (funcall (loc-reader loc)))
+(defun (setf ref) (new loc)
+  "Store a new value in the place referred to by a locative."
+  (funcall (loc-writer loc) new))
+
+(export 'with-locatives)
+(defmacro with-locatives (locs &body body)
+  "Evaluate BODY with implicit locatives.
+
+   LOCS is a list of items of the form (SYM [LOC-EXPR]), where SYM is a
+   symbol and LOC-EXPR evaluates to a locative.  If LOC-EXPR is omitted, it
+   defaults to SYM.  As an abbreviation for a common case, LOCS may be a
+   symbol instead of a list.
+
+   The BODY is evaluated in an environment where each SYM is a symbol macro
+   which expands to (ref LOC-EXPR) -- or, in fact, something similar which
+   doesn't break if LOC-EXPR has side-effects.  Thus, references, including
+   `setf' forms, fetch or modify the thing referred to by the LOC-EXPR.
+   Useful for covering over where something uses a locative."
+  (setf locs (mapcar (lambda (item)
+		       (cond ((atom item) (list item item))
+			     ((null (cdr item)) (list (car item) (car item)))
+			     (t item)))
+		     (if (listp locs) locs (list locs))))
+  (let ((tt (mapcar (lambda (l) (declare (ignore l)) (gensym)) locs))
+	(ll (mapcar #'cadr locs))
+	(ss (mapcar #'car locs)))
+    `(let (,@(mapcar (lambda (tmp loc) `(,tmp ,loc)) tt ll))
+       (symbol-macrolet (,@(mapcar (lambda (sym tmp)
+				     `(,sym (ref ,tmp))) ss tt))
+	 ,@body))))
+
+;;;--------------------------------------------------------------------------
 ;;; Anaphorics.
 
 (export 'it)