bug#56875: 29.0.50; [PATCH] Add thread-as macro

[Augusto Stoffel <arstoffel@gmail.com>]

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It has been noted many times that threading macros are not as useful in
Emacs Lisp because the APIs are not always consistent with regard to the
ordering of arguments.  A “thread-as” macro on the lines of Clojure's
'as->' [1] would address this difficulty.

The signature of the Clojure macro is (as-> expr name & forms), which
looks a bit odd to me.  Here I've changed this so the lexical variable
(name) comes before the initial value (expr).

Also, unlike thread-first and thread-last, in the attached version of
the macro there's no magic whereby an element of FORMS which is a
symbol, say fun, is turned into a function call, i.e. (fun name).  In
other words, (thread-as x 1 1+) returns the symbol 1+.  Clojure
behaves the same.  I'm not necessarily against changing the macro so
that the above example returns 2, but I don't see a good reason to do
so either; for one thing, the example gives a byte-compilation warning
(unused lexical variable x).

[1]: https://clojure.github.io/clojure/clojure.core-api.html#clojure.core/as-%3E


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From c044cf5891638cd0095db091e86303d683cf380d Mon Sep 17 00:00:00 2001
From: Augusto Stoffel <arstoffel@gmail.com>
Date: Mon, 1 Aug 2022 15:59:53 +0200
Subject: [PATCH] Add thread-as macro

* lisp/subr-x.el (thread-as): New macro.
---
 lisp/emacs-lisp/subr-x.el | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/lisp/emacs-lisp/subr-x.el b/lisp/emacs-lisp/subr-x.el
index d5d7bfeb6f..916ed39a03 100644
--- a/lisp/emacs-lisp/subr-x.el
+++ b/lisp/emacs-lisp/subr-x.el
@@ -81,6 +81,23 @@ thread-last
 threading."
   (declare (indent 0) (debug thread-first))
   `(internal--thread-argument nil ,@forms))
+
+(defmacro thread-as (var &rest forms)
+  "Successively bind VAR to the result of evaluating each of the FORMS.
+Return the last computed value.
+
+Example:
+     (thread-as x
+       4
+       (- 10 x)
+       (/ x 2))
+          ⇒ 3"
+  (declare (indent 1))
+  (if forms
+      `(let ((,var ,(car forms)))
+         (thread-as ,var ,@(cdr forms)))
+    var))
+
 (defsubst hash-table-empty-p (hash-table)
   "Check whether HASH-TABLE is empty (has 0 elements)."
   (zerop (hash-table-count hash-table)))
-- 
2.37.1