Add cl-with-accessors

* lisp/emacs-lisp/cl-macs.el (cl-with-accessors): New macro.
* doc/misc/cl.texi (Structures): Mention the new macro.
* test/lisp/emacs-lisp/cl-macs-tests.el (cl-lib-struct-with-accessors):
New Test.
* etc/NEWS (New macro 'cl-with-accessors'.): Mention the macro.

This macro is useful when making repeated use of a structures accessor
functions, such as reading from a slot and then writing to a slot.  It
is similar to 'with-slots' from EIEIO, but uses accessor functions
instead of slot names.

1 files changed, 44 insertions, 0 deletions

diff --git a/lisp/emacs-lisp/cl-macs.el b/lisp/emacs-lisp/cl-macs.el
index 960f2e6742b..cc1c6a6a5ad 100644
--- a/lisp/emacs-lisp/cl-macs.el
+++ b/lisp/emacs-lisp/cl-macs.el
@@ -2576,6 +2576,50 @@ See also `macroexp-let2'."
                           collect `(,(car name) ,gensym))
              ,@body)))))
 
+;;;###autoload
+(defmacro cl-with-accessors (bindings instance &rest body)
+  "Use BINDINGS as function calls on INSTANCE inside BODY.
+
+This macro helps when writing code that makes repeated use of the
+accessor functions of a structure or object instance, such as those
+created by `cl-defstruct' and `defclass'.
+
+BINDINGS is a list of (NAME ACCESSOR) pairs.  Inside BODY, NAME is
+treated as the function call (ACCESSOR INSTANCE) using
+`cl-symbol-macrolet'.  NAME can be used with `setf' and `setq' as a
+generalized variable.  Because of how the accessor is used,
+`cl-with-accessors' can be used with any generalized variable that can
+take a single argument, such as `car' and `cdr'.
+
+See also the macro `with-slots' described in the Info
+node `(eieio)Accessing Slots', which is similar, but uses slot names
+instead of accessor functions.
+
+\(fn ((NAME ACCESSOR) ...) INSTANCE &rest BODY)"
+  (declare (debug [(&rest (symbolp symbolp)) form body])
+           (indent 2))
+  (cond ((null body)
+         (macroexp-warn-and-return "`cl-with-accessors' used with empty body"
+                                   nil 'empty-body))
+        ((null bindings)
+         (macroexp-warn-and-return "`cl-with-accessors' used without accessors"
+                                   (macroexp-progn body)
+                                   'suspicious))
+        (t
+         (cl-once-only (instance)
+           (let ((symbol-macros))
+             (dolist (b bindings)
+               (pcase b
+                 (`(,(and (pred symbolp) var)
+                    ,(and (pred symbolp) accessor))
+                  (push `(,var (,accessor ,instance))
+                        symbol-macros))
+                 (_
+                  (error "Malformed `cl-with-accessors' binding: %S" b))))
+             `(cl-symbol-macrolet
+                  ,symbol-macros
+                ,@body))))))
+
 ;;; Multiple values.
 
 ;;;###autoload