diff options
Diffstat (limited to 'lisp/emacs-lisp/subr-x.el')
| -rw-r--r-- | lisp/emacs-lisp/subr-x.el | 22 |
1 files changed, 22 insertions, 0 deletions
diff --git a/lisp/emacs-lisp/subr-x.el b/lisp/emacs-lisp/subr-x.el index b90227da42f..a4514454c0b 100644 --- a/lisp/emacs-lisp/subr-x.el +++ b/lisp/emacs-lisp/subr-x.el @@ -389,6 +389,28 @@ it makes no sense to convert it to a string using (set-buffer source-buffer) (replace-buffer-contents tmp-buffer max-secs max-costs))))))))) +(defmacro named-let (name bindings &rest body) + "Looping construct taken from Scheme. +Like `let', bind variables in BINDINGS and then evaluate BODY, +but with the twist that BODY can evaluate itself recursively by +calling NAME, where the arguments passed to NAME are used +as the new values of the bound variables in the recursive invocation." + (declare (indent 2) (debug (symbolp (&rest (symbolp form)) body))) + (require 'cl-lib) + (let ((fargs (mapcar (lambda (b) (if (consp b) (car b) b)) bindings)) + (aargs (mapcar (lambda (b) (if (consp b) (cadr b))) bindings))) + ;; According to the Scheme semantics of named let, `name' is not in scope + ;; while evaluating the expressions in `bindings', and for this reason, the + ;; "initial" function call below needs to be outside of the `cl-labels'. + ;; When the "self-tco" eliminates all recursive calls, the `cl-labels' + ;; expands to a lambda which the byte-compiler then combines with the + ;; funcall to make a `let' so we end up with a plain `while' loop and no + ;; remaining `lambda' at all. + `(funcall + (cl-labels ((,name ,fargs . ,body)) #',name) + . ,aargs))) + + (provide 'subr-x) ;;; subr-x.el ends here |