>From 250b999e094ccdf11e37bb80f2d6a41b64b96dfd 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 `thunk-let' and `thunk-let*'

* lisp/emacs-lisp/thunk.el (thunk-let, thunk-let*): New macros.
* test/lisp/emacs-lisp/thunk-tests.el:
(thunk-let-basic-test, thunk-let*-basic-test)
(thunk-let-bound-vars-cant-be-set-test)
(thunk-let-laziness-test, thunk-let*-laziness-test)
(thunk-let-bad-binding-test): New tests for `thunk-let' and
`thunk-let*.
---
 etc/NEWS                            |  4 +++
 lisp/emacs-lisp/thunk.el            | 56 +++++++++++++++++++++++++++++++++++++
 test/lisp/emacs-lisp/thunk-tests.el | 50 +++++++++++++++++++++++++++++++++
 3 files changed, 110 insertions(+)

diff --git a/etc/NEWS b/etc/NEWS
index c47ca42d27..bfa8be39f9 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -109,6 +109,10 @@ Snake and Pong are more playable on HiDPI displays.
 *** Completing filenames in the minibuffer via 'C-TAB' now uses the
 styles as configured by the variable 'completion-styles'.
 
+** Thunk
+*** The new macros 'thunk-let' and 'thunk-let*' are analogue to `let'
+and `let*' but create bindings that are evaluated lazily.
+
 
 * New Modes and Packages in Emacs 27.1
 
diff --git a/lisp/emacs-lisp/thunk.el b/lisp/emacs-lisp/thunk.el
index 371d10444b..9cb16857f4 100644
--- a/lisp/emacs-lisp/thunk.el
+++ b/lisp/emacs-lisp/thunk.el
@@ -41,6 +41,10 @@
 ;; following:
 ;;
 ;;    (thunk-force delayed)
+;;
+;; This file also defines macros `thunk-let' and `thunk-let*' that are
+;; analogous to `let' and `let*' but provide lazy evaluation of
+;; bindings by using thunks implicitly (i.e. in the expansion).
 
 ;;; Code:
 
@@ -71,5 +75,57 @@ thunk-evaluated-p
   "Return non-nil if DELAYED has been evaluated."
   (funcall delayed t))
 
+(defmacro thunk-let (bindings &rest body)
+  "Like `let' but create lazy bindings.
+
+BINDINGS is a list of elements of the form (SYMBOL EXPRESSION).
+Any binding EXPRESSION is not evaluated before the variable
+SYMBOL is used for the first time when evaluating the BODY.
+
+It is not allowed to set `thunk-let' or `thunk-let*' bound
+variables.
+
+Using `thunk-let' and `thunk-let*' requires `lexical-binding'."
+  (declare (indent 1) (debug let))
+  (cl-callf2 mapcar
+      (lambda (binding)
+        (pcase binding
+          (`(,(pred symbolp) ,_) binding)
+          (_ (signal 'error (cons "Bad binding in thunk-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 thunk-let* (bindings &rest body)
+  "Like `let*' but create lazy bindings.
+
+BINDINGS is a list of elements of the form (SYMBOL EXPRESSION).
+Any binding EXPRESSION is not evaluated before the variable
+SYMBOL is used for the first time when evaluating the BODY.
+
+It is not allowed to set `thunk-let' or `thunk-let*' bound
+variables.
+
+Using `thunk-let' and `thunk-let*' requires `lexical-binding'."
+  (declare (indent 1) (debug let))
+  (cl-reduce
+   (lambda (expr binding) `(thunk-let (,binding) ,expr))
+   (nreverse bindings)
+   :initial-value (macroexp-progn body)))
+
+
 (provide 'thunk)
 ;;; thunk.el ends here
diff --git a/test/lisp/emacs-lisp/thunk-tests.el b/test/lisp/emacs-lisp/thunk-tests.el
index 973a14b818..e122098b16 100644
--- a/test/lisp/emacs-lisp/thunk-tests.el
+++ b/test/lisp/emacs-lisp/thunk-tests.el
@@ -51,5 +51,55 @@
     (thunk-force thunk)
     (should (= x 1))))
 
+
+
+;; thunk-let tests
+
+(ert-deftest thunk-let-basic-test ()
+  "Test whether bindings are established."
+  (should (equal (thunk-let ((x 1) (y 2)) (+ x y)) 3)))
+
+(ert-deftest thunk-let*-basic-test ()
+  "Test whether bindings are established."
+  (should (equal (thunk-let* ((x 1) (y (+ 1 x))) (+ x y)) 3)))
+
+(ert-deftest thunk-let-bound-vars-cant-be-set-test ()
+  "Test whether setting a `thunk-let' bound variable fails."
+  (should-error
+   (eval '(thunk-let ((x 1)) (let ((y 7)) (setq x (+ x y)) (* 10 x))) t)))
+
+(ert-deftest thunk-let-laziness-test ()
+  "Test laziness of `thunk-let'."
+  (should
+   (equal (let ((x-evalled nil)
+                (y-evalled nil))
+            (thunk-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 thunk-let*-laziness-test ()
+  "Test laziness of `thunk-let*'."
+  (should
+   (equal (let ((x-evalled nil)
+                (y-evalled nil)
+                (z-evalled nil)
+                (a-evalled nil))
+            (thunk-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 thunk-let-bad-binding-test ()
+  "Test whether a bad binding causes a compiler error."
+  (should-error (byte-compile (thunk-let ((x 1 1)) x)))
+  (should-error (byte-compile (thunk-let (27) x)))
+  (should-error (byte-compile (thunk-let x x))))
+
+
 (provide 'thunk-tests)
 ;;; thunk-tests.el ends here
-- 
2.15.0