humble proposal: New special form progn-1

[Tino Calancha <tino.calancha@gmail.com>]

Dear all,

i got this idea few days ago (see the patch at the end).

Even though I envision that no one here will like this proposal,
for me it's instructive to learn from your answers why this is
not a good idea.

So, why is this a bad idea?

Thanks in advance for your answer,
Tino


My (weak) motivation for introduce this is:

* Compact (and familiar) syntaxis.
* Same reasons to exists as prog2 has (excluding historical reasons).
* Other way to acomplish one usual task.
* Allow lower indentation level (see below):

(progn-1
  (form1)
  (form2)
  (form3)
  .
  .
  .
  (formN-1)
  (formN))

(prog1
     (progn
       (form1)
       (form2)
       (form3)
       .
       .
       .
       (formN-1))
   (formN))

(prog2
     (progn
       (form1)
       (form2)
       (form3)
       .
       .
       .
       (formN-2))
     (prog1
       (formN-1)
       (formN)))

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From c9b38c42b253ca004c3aef4dd9dde274aa717d91 Mon Sep 17 00:00:00 2001
From: Tino Calancha <tino.calancha@gmail.com>
Date: Wed, 27 Jul 2016 20:40:25 +0900
Subject: [PATCH] New special form progn-1

* src/eval.c (progn-1): Eval sequentially N forms and
return the value of the form N-1.
---
  src/eval.c | 18 ++++++++++++++++++
  1 file changed, 18 insertions(+)

diff --git a/src/eval.c b/src/eval.c
index 33b82f7..5eb0cfd 100644
--- a/src/eval.c
+++ b/src/eval.c
@@ -431,6 +431,23 @@ usage: (progn BODY...)  */)
    return val;
  }

+DEFUN ("progn-1", Fprogn_1, Sprogn_1, 2, UNEVALLED, 0,
+       doc: /* Eval N forms sequentially; return value from form N-1.
+usage: (progn-1 FORM_1 FORM_2...FORM_N-1 FORM_N)  */)
+  (Lisp_Object body)
+{
+  Lisp_Object val = Qnil;
+
+  while (CONSP (XCDR (body)))
+    {
+      val = eval_sub (XCAR (body));
+      body = XCDR (body);
+    }
+  eval_sub (XCAR (body));
+
+  return val;
+}
+
  /* Evaluate BODY sequentially, discarding its value.  Suitable for
     record_unwind_protect.  */

@@ -3906,6 +3923,7 @@ alist of active lexical bindings.  */);
    defsubr (&Sif);
    defsubr (&Scond);
    defsubr (&Sprogn);
+  defsubr (&Sprogn_1);
    defsubr (&Sprog1);
    defsubr (&Sprog2);
    defsubr (&Ssetq);
-- 
2.8.1

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In GNU Emacs 25.1.50.1 (x86_64-pc-linux-gnu, GTK+ Version 3.20.6)
  of 2016-07-27 built
Repository revision: e0d425976e3a83585db9a586687897fe1ac6455f