GNU bug report logs - #28254
26.0.50; SRFI-2 and-let*

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Message #23 received at 28254 <at> debbugs.gnu.org (full text, mbox):

From: Mark Oteiza <mvoteiza <at> udel.edu>
To: Michael Heerdegen <michael_heerdegen <at> web.de>
Cc: 28254 <at> debbugs.gnu.org, npostavs <at> users.sourceforge.net
Subject: Re: bug#28254: 26.0.50; SRFI-2 and-let*
Date: Sat, 2 Sep 2017 09:36:04 -0400

On 02/09/17 at 07:25am, Michael Heerdegen wrote:
>Mark Oteiza <mvoteiza <at> udel.edu> writes:
>
>> >> I didn't try writing it in the style of if-let*--perhaps if-let* could
>> >> be extended and all three macros would learn (EXPR).
>> >
>> >Yes, I think it's best if all foo-let* macros interpret the varlist in
>> >the same way.
>>
>> Alright, I'll look at it.
>
>Isn't there a problem with EXPR being a symbol S, which already has a
>different meaning (bind S to nil)?  Though, this seems barely useful to
>me.  Anyway, introducing (EXPR) would thus be backward incompatible.

Yeah, that is true.  The following patch implements most of the
previous, except it doesn't specially handle EXPR being just a symbol.
All the incumbent subr-x-tests pass.

This single tuple special case is troublesome IMO:

 (if-let* (x) "dogs" "cats") => "cats"
 (if-let* (x (y 2)) "dogs" "cats") => (void-function y)
 (if-let* (x (y 1) (z 2)) "dogs" "cats") => "cats"

I'm curious if this was brought up in the old discussion when this was
implemented.

diff --git a/lisp/emacs-lisp/subr-x.el b/lisp/emacs-lisp/subr-x.el
index 849ac19d6a..eeacdbcfcd 100644
--- a/lisp/emacs-lisp/subr-x.el
+++ b/lisp/emacs-lisp/subr-x.el
@@ -83,10 +83,16 @@ thread-last
  `(internal--thread-argument nil ,@forms))

(defsubst internal--listify (elt)
-  "Wrap ELT in a list if it is not one."
-  (if (not (listp elt))
-      (list elt)
-    elt))
+  "Wrap ELT in a list if it is not one.
+If ELT is of the form ((EXPR)), listify (EXPR) with a dummy symbol."
+  (cond
+   ((nlistp elt) (list elt))
+   ((atom (car elt)) elt)
+   ((and (null (cdr elt))
+         (let ((form (car elt)))
+           (or (listp form) (atom form))))
+    (list (cl-gensym) (car elt)))
+   (t elt)))

(defsubst internal--check-binding (binding)
  "Check BINDING is properly formed."
@@ -122,8 +128,11 @@ if-let*
Each binding is evaluated in turn with `let*', and evaluation
stops if a binding value is nil.  If all are non-nil, the value
of THEN is returned, or the last form in ELSE is returned.
+
Each element of VARLIST is a symbol (which is bound to nil)
or a list (SYMBOL VALUEFORM) (which binds SYMBOL to the value of VALUEFORM).
+An element can additionally be of the form (EXPR), which is
+evaluated and checked for nil.
In the special case you only want to bind a single value,
VARLIST can just be a plain tuple.
\n(fn VARLIST THEN ELSE...)"
@@ -134,18 +143,23 @@ if-let*
             (not (listp (car bindings))))
    ;; Adjust the single binding case
    (setq bindings (list bindings)))
-  `(let* ,(internal--build-bindings bindings)
-     (if ,(car (internal--listify (car (last bindings))))
-         ,then
-       ,@else)))
+  (if bindings
+      `(let* ,(setq bindings (internal--build-bindings bindings))
+         (if ,(caar (last bindings))
+             ,then
+           ,@else))
+    `(let* () ,then)))

(defmacro when-let* (bindings &rest body)
  "Bind variables according to VARLIST and conditionally eval BODY.
Each binding is evaluated in turn with `let*', and evaluation
stops if a binding value is nil.  If all are non-nil, the value
of the last form in BODY is returned.
+
Each element of VARLIST is a symbol (which is bound to nil)
or a list (SYMBOL VALUEFORM) (which binds SYMBOL to the value of VALUEFORM).
+An element can additionally be of the form (EXPR), which is
+evaluated and checked for nil.
In the special case you only want to bind a single value,
VARLIST can just be a plain tuple.
\n(fn VARLIST BODY...)"
@@ -154,7 +168,12 @@ when-let*

(defalias 'if-let 'if-let*)
(defalias 'when-let 'when-let*)
-(defalias 'and-let* 'when-let*)
+
+(defmacro and-let* (varlist &rest body)
+  "Bind variables according to VARLIST and conditionally eval BODY.
+Like `when-let*', except if BODY is empty and all the bindings
+are non-nil, then the result is t."
+  `(when-let* ,varlist ,@(or body '(t))))

(defsubst hash-table-empty-p (hash-table)
  "Check whether HASH-TABLE is empty (has 0 elements)."

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