base: New macro setf-default.

[lisp] / mdw-base.lisp

diff --git a/mdw-base.lisp b/mdw-base.lisp
index e87c51191ab1e155c07758839b8b1c9ba26c7efc..2c463e980aad8d6547c08728638e0912235cea61 100644 (file)
--- a/mdw-base.lisp
+++ b/mdw-base.lisp
@@ -28,12 +28,13 @@
 
 (defpackage #:mdw.base
   (:use #:common-lisp)
-  (:export #:compile-time-defun
+  (:export #:unsigned-fixnum
+          #:compile-time-defun
           #:show
           #:stringify #:mappend #:listify #:fix-pair #:pairify #:parse-body
           #:whitespace-char-p
           #:slot-uninitialized
-          #:nlet #:while #:until #:case2 #:ecase2
+          #:nlet #:while #:until #:case2 #:ecase2 #:setf-default
           #:with-gensyms #:let*/gensyms #:with-places
           #:locp #:locf #:ref #:with-locatives
           #:update-place #:update-place-after
@@ -43,13 +44,20 @@ (defpackage #:mdw.base
 
 (in-package #:mdw.base)
 
+;;;--------------------------------------------------------------------------
+;;; Useful types.
+
+(deftype unsigned-fixnum ()
+  "Unsigned fixnums; useful as array indices and suchlike."
+  `(mod ,most-positive-fixnum))
+
 ;;;--------------------------------------------------------------------------
 ;;; Some simple macros to get things going.
 
 (defmacro compile-time-defun (name args &body body)
   "Define a function which can be used by macros during the compilation
    process."
-  `(eval-when (:compile-toplevel :load-toplevel)
+  `(eval-when (:compile-toplevel :load-toplevel :execute)
      (defun ,name ,args ,@body)))
 
 (defmacro show (x)
@@ -111,7 +119,9 @@ (compile-time-defun pairify (x &optional (y nil defaultp))
 (defun whitespace-char-p (ch)
   "Return whether CH is a whitespace character or not."
   (case ch
-    ((#\space #\tab #\newline #\return #\vt #\formfeed) t)
+    ((#\space #\tab #\newline #\return #\vt
+             #+cmu #\formfeed
+             #+clisp #\page) t)
     (t nil)))
 
 (declaim (ftype (function nil ()) slot-unitialized))
@@ -229,6 +239,31 @@ (defmacro ecase2 (vform &body clauses)
   "Like `case2', but signals an error if no clause matches the SCRUTINEE."
   (do-case2-like 'ecase vform clauses))
 
+(defmacro setf-default (&rest specs &environment env)
+  "Like setf, but only sets places which are currently nil.
+
+   The arguments are an alternating list of PLACEs and DEFAULTs.  If a PLACE
+   is nil, the DEFAULT is evaluated and stored in the PLACE; otherwise the
+   default is /not/ stored.  The result is the (new) value of the last
+   PLACE."
+  (labels ((doit (specs)
+            (cond ((null specs) nil)
+                  ((null (cdr specs))
+                   (error "Odd number of arguments for SETF-DEFAULT."))
+                  (t
+                   (let ((place (car specs))
+                         (default (cadr specs))
+                         (rest (cddr specs)))
+                     (multiple-value-bind
+                         (vars vals store-vals writer reader)
+                         (get-setf-expansion place env)
+                       `(let* ,(mapcar #'list vars vals)
+                          (or ,reader
+                              (multiple-value-bind ,store-vals ,default
+                                ,writer))
+                          ,@(and rest (list (doit rest))))))))))
+    (doit specs)))
+
 ;;;--------------------------------------------------------------------------
 ;;; with-places