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* bug#48212: [PATCH] subr-x.el: Added `while-let` and `while-let*`
@ 2021-05-03 23:29 Hu Lucius
  0 siblings, 0 replies; only message in thread
From: Hu Lucius @ 2021-05-03 23:29 UTC (permalink / raw)
  To: 48212

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Hi,

I've added two functions in `subr-x`, `while-let` and `while-let*`.

`(while-let SPEC BODY)` evaluate each binding in turn and stop if a
binding value is nil.
BODY is evaluated repeatedly until a binding value is nil.

`(while-let* VARLIST BODY)` is like `while-let` except VARLIST is of
the form ((SYMBOL SOMETHING) ...).

Example:
Suppose we've a list, (setq xs '(1 2 3 4 5)), the following while-let form

(while-let ((x (pop xs)))
   (print x))

is equivalent to

(while (when-let ((x (pop xs)))
             (print x)
             t))

The last expression `t` is added so that regardless of the return value
of the last form in BODY, the while loop continues, unless a binding
value is nil.

Signed-off-by: Lucius Hu <lebensterben@users.noreply.github.com>
---
 lisp/emacs-lisp/subr-x.el | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/lisp/emacs-lisp/subr-x.el b/lisp/emacs-lisp/subr-x.el
index 9c8c967ee9..3203cce455 100644
--- a/lisp/emacs-lisp/subr-x.el
+++ b/lisp/emacs-lisp/subr-x.el
@@ -156,6 +156,13 @@ are non-nil, then the result is non-nil."
              ,@(or body `(,res))))
       `(let* () ,@(or body '(t))))))

+(defmacro while-let* (varlist &rest body)
+  "Bind variables according to VARLIST and conditionally evaluate BODY.
+This is like `while-let' but doesn't handle a VARLIST of the form
+\(SYMBOL SOMETHING) sepcially."
+  (declare (indent 1) (debug if-let*))
+  (list 'while (list 'if-let* varlist (nconc (macroexp-progn body) (list
t)))))
+
 ;;;###autoload
 (defmacro if-let (spec then &rest else)
   "Bind variables according to SPEC and evaluate THEN or ELSE.
@@ -193,6 +200,15 @@ The variable list SPEC is the same as in `if-let'."
   (declare (indent 1) (debug if-let))
   (list 'if-let spec (macroexp-progn body)))

+;;;###autoload
+(defmacro while-let (spec &rest body)
+  "Bind variables according to SPEC and conditionally evaluate BODY.
+Evaluate each binding in turn, stopping if a binding value is nil.
+BODY is repeatedly evaluated until a binding value is nil.
+The return value is always nil."
+  (declare (indent 1) (debug if-let))
+  (list 'while (list 'if-let spec (nconc (macroexp-progn body) (list t)))))
+
 (defsubst hash-table-empty-p (hash-table)
   "Check whether HASH-TABLE is empty (has 0 elements)."
   (zerop (hash-table-count hash-table)))
-- 
2.31.1

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2021-05-03 23:29 bug#48212: [PATCH] subr-x.el: Added `while-let` and `while-let*` Hu Lucius

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