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*.

* doc/lispref/eval.texi (Deferred Eval): New section.
* doc/lispref/elisp.texi: Update menu.

5 files changed, 232 insertions, 5 deletions

diff --git a/doc/lispref/elisp.texi b/doc/lispref/elisp.texi
index c7525945845..a271749e044 100644
--- a/doc/lispref/elisp.texi
+++ b/doc/lispref/elisp.texi
@@ -455,6 +455,7 @@ Evaluation
                               the program).
 * Backquote::               Easier construction of list structure.
 * Eval::                    How to invoke the Lisp interpreter explicitly.
+* Deferred Eval::           Deferred and lazy evaluation of forms.
 
 Kinds of Forms
 
diff --git a/doc/lispref/eval.texi b/doc/lispref/eval.texi
index 064fca22ff5..74fefdb71bc 100644
--- a/doc/lispref/eval.texi
+++ b/doc/lispref/eval.texi
@@ -20,11 +20,12 @@ function @code{eval}.
 
 @ifnottex
 @menu
-* Intro Eval::  Evaluation in the scheme of things.
-* Forms::       How various sorts of objects are evaluated.
-* Quoting::     Avoiding evaluation (to put constants in the program).
-* Backquote::   Easier construction of list structure.
-* Eval::        How to invoke the Lisp interpreter explicitly.
+* Intro Eval::     Evaluation in the scheme of things.
+* Forms::          How various sorts of objects are evaluated.
+* Quoting::        Avoiding evaluation (to put constants in the program).
+* Backquote::      Easier construction of list structure.
+* Eval::           How to invoke the Lisp interpreter explicitly.
+* Deferred Eval::  Deferred and lazy evaluation of forms.
 @end menu
 
 @node Intro Eval
@@ -877,3 +878,115 @@ particular elements, like this:
 @end group
 @end example
 @end defvar
+
+@node Deferred Eval
+@section Deferred and Lazy Evaluation
+
+@cindex deferred evaluation
+@cindex lazy evaluation
+
+
+  Sometimes it is useful to delay the evaluation of an expression, for
+example if you want to avoid to perform a time-consuming calculation
+in the case that it turns out that the result is not needed in the
+future of the program.  Therefore, the @file{thunk} library provides
+the following functions and macros:
+
+@cindex thunk
+@defmac thunk-delay forms@dots{}
+Return a @dfn{thunk} for evaluating the @var{forms}.  A thunk is a
+closure (@pxref{Closures}) that inherits the lexical enviroment of the
+@code{thunk-delay} call.  Using this macro requires
+@code{lexical-binding}.
+@end defmac
+
+@defun thunk-force thunk
+Force @var{thunk} to perform the evaluation of the forms specified in
+the @code{thunk-delay} that created the thunk.  The result of the
+evaluation of the last form is returned.  The @var{thunk} also
+``remembers'' that it has been forced: Any further calls of
+@code{thunk-force} with the same @var{thunk} will just return the same
+result without evaluating the forms again.
+@end defun
+
+@defmac thunk-let (bindings@dots{}) forms@dots{}
+This macro is analogous to @code{let} but creates ``lazy'' variable
+bindings.  Any binding has the form @w{@code{(@var{symbol}
+@var{value-form})}}.  Unlike @code{let}, the evaluation of any
+@var{value-form} is deferred until the binding of the according
+@var{symbol} is used for the first time when evaluating the
+@var{forms}.  Any @var{value-form} is evaluated at most once.  Using
+this macro requires @code{lexical-binding}.
+@end defmac
+
+Example:
+
+@example
+@group
+(defun f (number)
+  (thunk-let ((derived-number
+              (progn (message "Calculating 1 plus 2 times %d" number)
+                     (1+ (* 2 number)))))
+    (if (> number 10)
+        derived-number
+      number)))
+@end group
+
+@group
+(f 5)
+@result{} 5
+@end group
+
+@group
+(f 12)
+@print{} Calculating 1 plus 2 times 12
+@result{} 25
+@end group
+
+@end example
+
+Because of the special nature of lazily bound variables, it is an error
+to set them (e.g.@: with @code{setq}).
+
+
+@defmac thunk-let* (bindings@dots{}) forms@dots{}
+This is like @code{thunk-let} but any expression in @var{bindings} is allowed
+to refer to preceding bindings in this @code{thunk-let*} form.  Using
+this macro requires @code{lexical-binding}.
+@end defmac
+
+@example
+@group
+(thunk-let* ((x (prog2 (message "Calculating x...")
+                    (+ 1 1)
+                  (message "Finished calculating x")))
+             (y (prog2 (message "Calculating y...")
+                    (+ x 1)
+                  (message "Finished calculating y")))
+             (z (prog2 (message "Calculating z...")
+                    (+ y 1)
+                  (message "Finished calculating z")))
+             (a (prog2 (message "Calculating a...")
+                    (+ z 1)
+                  (message "Finished calculating a"))))
+  (* z x))
+
+@print{} Calculating z...
+@print{} Calculating y...
+@print{} Calculating x...
+@print{} Finished calculating x
+@print{} Finished calculating y
+@print{} Finished calculating z
+@result{} 8
+
+@end group
+@end example
+
+@code{thunk-let} and @code{thunk-let*} use thunks implicitly: their
+expansion creates helper symbols and binds them to thunks wrapping the
+binding expressions.  All references to the original variables in the
+body @var{forms} are then replaced by an expression that calls
+@code{thunk-force} with the according helper variable as the argument.
+So, any code using @code{thunk-let} or @code{thunk-let*} could be
+rewritten to use thunks, but in many cases using these macros results
+in nicer code than using thunks explicitly.
diff --git a/etc/NEWS b/etc/NEWS
index c47ca42d277..6b3e7fc244b 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'.
 
+** New macros 'thunk-let' and 'thunk-let*'.
+These macros 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 371d10444b2..895fa86722d 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 +75,60 @@ with the same DELAYED argument."
   "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)))
+
+;; (defalias 'lazy-let  #'thunk-let)
+;; (defalias 'lazy-let* #'thunk-let*)
+
+
 (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 973a14b8180..a63ce289e8a 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 an error when expanding."
+  (should-error (macroexpand '(thunk-let ((x 1 1)) x)))
+  (should-error (macroexpand '(thunk-let (27) x)))
+  (should-error (macroexpand '(thunk-let x x))))
+
+
 (provide 'thunk-tests)
 ;;; thunk-tests.el ends here