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

[Augusto Stoffel]

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

0001-Add-thread-as-macro.patch
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