Re: [SUSPECTED SPAM] Re: `thunk-let'?

[Michael Heerdegen]

Stefan Monnier <address@hidden> writes:

> > Ok, so let's be brave and aim to add it as `lazy-let' to subr-x.
> > There is a question I want us to think about: what should the semantics
> > of a `lazy-let' bound variable be if it is bound or set inside the
> > BODY?
>
> The semantics of `setq`ing such a var should be: compile-time error.
> The semantics of let-rebinding such a variable should be for the new
> binding to hide the outer (lazy) one.

Is this patch acceptable (also attached as file)?

>From 4941becd07f6ffbe387006248193d95b258be526 Mon Sep 17 00:00:00 2001
From: Michael Heerdegen <address@hidden>
Date: Thu, 2 Nov 2017 18:45:34 +0100
Subject: [PATCH] Add macros `lazy-let' and `lazy-let*'

* lisp/emacs-lisp/subr-x.el (lazy-let, lazy-let*): New macros.
* lisp/emacs-lisp/thunk.el (thunk-delay, thunk-force): Add autoload
cookies.
* test/lisp/emacs-lisp/subr-x-tests.el: Use lexical-binding.
(subr-x-lazy-let-basic-test, subr-x-lazy-let*-basic-test)
(subr-x-lazy-let-bound-vars-cant-be-bound-test)
(subr-x-lazy-let-lazyness-test, subr-x-lazy-let*-lazyness-test)
(subr-x-lazy-let-bad-binding-test): New tests for `lazy-let' and `lazy-let*.
---
 etc/NEWS                             |  4 +++
 lisp/emacs-lisp/subr-x.el            | 48 ++++++++++++++++++++++++++++++++++
 lisp/emacs-lisp/thunk.el             |  2 ++
 test/lisp/emacs-lisp/subr-x-tests.el | 50 +++++++++++++++++++++++++++++++++++-
 4 files changed, 103 insertions(+), 1 deletion(-)

diff --git a/etc/NEWS b/etc/NEWS
index c47ca42d27..8b1f659ebf 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -128,6 +128,10 @@ calling 'eldoc-message' directly.
 
 * Lisp Changes in Emacs 27.1
 
+---
+** The new macros 'lazy-let' and 'lazy-let*' are analogue to `let' and
+`let*' but create bindings that are evaluated lazily.
+
 ---
 ** The 'file-system-info' function is now available on all platforms.
 instead of just Microsoft platforms.  This fixes a 'get-free-disk-space'
diff --git a/lisp/emacs-lisp/subr-x.el b/lisp/emacs-lisp/subr-x.el
index 8ed29d8659..ce8956c96f 100644
--- a/lisp/emacs-lisp/subr-x.el
+++ b/lisp/emacs-lisp/subr-x.el
@@ -245,6 +245,54 @@ string-remove-suffix
       (substring string 0 (- (length string) (length suffix)))
     string))
 
+(defmacro lazy-let (bindings &rest body)
+  "Like `let' but make delayed bindings.
+
+This is like `let' but any binding expression is not evaluated
+before the variable is used for the first time.
+
+It is not allowed to set `lazy-let' or `lazy-let*' bound
+variables."
+  (declare (indent 1) (debug let))
+  (cl-callf2 mapcar
+      (lambda (binding)
+        (pcase binding
+          ((or (and (pred symbolp) s)
+               `(,(and (pred symbolp) s)))
+           `(,s nil))
+          (`(,(pred symbolp) ,_) binding)
+          (_ (signal 'error (cons "Bad binding in lazy-let"
+                                  (list binding))))))
+      bindings)
+  (cl-callf2 mapcar
+      (pcase-lambda (`(,var ,binding))
+        (list (make-symbol (concat (symbol-name var) "-thunk"))
+              var binding))
+      bindings)
+  `(let ,(mapcar
+          (pcase-lambda (`(,thunk-var ,_var ,binding))
+            `(,thunk-var (thunk-delay ,binding)))
+          bindings)
+     (cl-symbol-macrolet
+         ,(mapcar (pcase-lambda (`(,thunk-var ,var ,_binding))
+                    `(,var (thunk-force ,thunk-var)))
+                  bindings)
+       ,@body)))
+
+(defmacro lazy-let* (bindings &rest body)
+  "Like `let*' but make delayed bindings.
+
+This is like `let*' but any binding expression is not evaluated
+before the variable is used for the first time.
+
+It is not allowed to set `lazy-let' or `lazy-let*' bound
+variables."
+  (declare (indent 1) (debug let))
+  (cl-reduce
+   (lambda (expr binding) `(lazy-let (,binding) ,expr))
+   (nreverse bindings)
+   :initial-value `(progn ,@body)))
+
 (provide 'subr-x)
 
 ;;; subr-x.el ends here
diff --git a/lisp/emacs-lisp/thunk.el b/lisp/emacs-lisp/thunk.el
index 371d10444b..0c5d0b709e 100644
--- a/lisp/emacs-lisp/thunk.el
+++ b/lisp/emacs-lisp/thunk.el
@@ -46,6 +46,7 @@
 
 (eval-when-compile (require 'cl-macs))
 
+;;;###autoload
 (defmacro thunk-delay (&rest body)
   "Delay the evaluation of BODY."
   (declare (debug t))
@@ -61,6 +62,7 @@ thunk-delay
              (setf ,forced t))
            ,val)))))
 
+;;;###autoload
 (defun thunk-force (delayed)
   "Force the evaluation of DELAYED.
 The result is cached and will be returned on subsequent calls
diff --git a/test/lisp/emacs-lisp/subr-x-tests.el 
b/test/lisp/emacs-lisp/subr-x-tests.el
index 0e8871d9a9..c477a63a29 100644
--- a/test/lisp/emacs-lisp/subr-x-tests.el
+++ b/test/lisp/emacs-lisp/subr-x-tests.el
@@ -1,4 +1,4 @@
-;;; subr-x-tests.el --- Testing the extended lisp routines
+;;; subr-x-tests.el --- Testing the extended lisp routines    -*- 
lexical-binding: t -*-
 
 ;; Copyright (C) 2014-2017 Free Software Foundation, Inc.
 
@@ -538,6 +538,54 @@
                    (format "abs sum is: %s"))
                  "abs sum is: 15")))
 
+
+;; lazy-let tests
+
+(ert-deftest subr-x-lazy-let-basic-test ()
+  "Test whether bindings are established."
+  (should (equal (lazy-let ((x 1) (y 2)) (+ x y)) 3)))
+
+(ert-deftest subr-x-lazy-let*-basic-test ()
+  "Test whether bindings are established."
+  (should (equal (lazy-let* ((x 1) (y (+ 1 x))) (+ x y)) 3)))
+
+(ert-deftest subr-x-lazy-let-bound-vars-cant-be-bound-test ()
+  "Test whether setting or binding a `lazy-let' bound variable fails."
+  (should-error (eval '(lazy-let ((x 1)) (let ((y 7)) (setq x (+ x y)) (* 10 
x))) t))
+  (should-error (eval '(lazy-let ((x 1)) (let ((x 2)) x)) t)))
+
+(ert-deftest subr-x-lazy-let-lazyness-test ()
+  "Test for lazyness."
+  (should
+   (equal (let ((x-evalled nil)
+                (y-evalled nil))
+            (lazy-let ((x (progn (setq x-evalled t) (+ 1 2)))
+                       (y (progn (setq y-evalled t) (+ 3 4))))
+              (let ((evalled-y y))
+                (list x-evalled y-evalled evalled-y))))
+          (list nil t 7))))
+
+(ert-deftest subr-x-lazy-let*-lazyness-test ()
+  "Test lazyness of `lazy-let*'."
+  (should
+   (equal (let ((x-evalled nil)
+                (y-evalled nil)
+                (z-evalled nil)
+                (a-evalled nil))
+            (lazy-let* ((x (progn (setq x-evalled t) (+ 1 1)))
+                        (y (progn (setq y-evalled t) (+ x 1)))
+                        (z (progn (setq z-evalled t) (+ y 1)))
+                        (a (progn (setq a-evalled t) (+ z 1))))
+              (let ((evalled-z z))
+                (list x-evalled y-evalled z-evalled a-evalled evalled-z))))
+          (list t t t nil 4))))
+
+(ert-deftest subr-x-lazy-let-bad-binding-test ()
+  "Test whether a bad binding causes a compiler error."
+  (should-error (byte-compile (lazy-let ((x 1 1)) x)))
+  (should-error (byte-compile (lazy-let (27) x)))
+  (should-error (byte-compile (lazy-let x x))))
+
 
 (provide 'subr-x-tests)
 ;;; subr-x-tests.el ends here
-- 
2.14.2

0001-Add-macros-lazy-let-and-lazy-let.patch
Description: Text Data

Thanks,

Michael.