Re: [PATCH] * src/eval.c: Stop checking for nvars, and use only CONSP

[Naoya Yamashita]

> The patch looks fine, thank you.

Thanks!


> Just one detail about the tests: the change you made affects the
> evaluation of `let` in the case where the code is interpreted but it
> is not used when the code is byte-compiled.  So you might like your
> tests to use things like
> 
>     (eval '(let ...) t)
> 
> to avoid the compiler getting in the way.

Thanks, I didn't notice this point!  I fix testcases.


> Also, while this `let` is indeed invalid code, I don't think we
> guarantee that it will signal an error, and especially not at runtime
> (it's more likely to signal an error at macroexpansion or compile
> time).
> 
> I think the compiler (or `macroexpand-all`) should make an effort to
> detect and diagnose those problems, but I don't think it's important to
> catch those problems in the interpreter.

That testcase comes from this code (src/eval.c:L1014) which we already had.

      else if (! NILP (Fcdr (Fcdr (elt))))
        signal_error ("`let' bindings can have only one value-form", elt);

I tried to remove this, my temp Emacs works like this in *scratch* buffer.

    (let ((a 1 2))
       a)                       ; Type C-j
    1

This is very strange I think.  I still think it's important for
Emacs, even as an interpreter, to produce errors.

>From 8e65fea2d95bf7118ecd2b816f3b49292353a431 Mon Sep 17 00:00:00 2001
From: Naoya Yamashita <conao3@gmail.com>
Date: Tue, 2 Mar 2021 10:17:29 +0900
Subject: [PATCH] * src/eval.c: Stop checking for nvars, and use only CONSP

* src/eval.c (let): Remove checking nvars (length of arglist),
and use only CONSP check.
---
 src/eval.c             |  6 ++----
 test/src/eval-tests.el | 33 +++++++++++++++++++++++++++++++++
 2 files changed, 35 insertions(+), 4 deletions(-)

diff --git a/src/eval.c b/src/eval.c
index 542d7f686e..30783f204a 100644
--- a/src/eval.c
+++ b/src/eval.c
@@ -1001,11 +1001,10 @@ DEFUN ("let", Flet, Slet, 1, UNEVALLED, 0,
   /* Make space to hold the values to give the bound variables.  */
   EMACS_INT varlist_len = list_length (varlist);
   SAFE_ALLOCA_LISP (temps, varlist_len);
-  ptrdiff_t nvars = varlist_len;
 
   /* Compute the values and store them in `temps'.  */
 
-  for (argnum = 0; argnum < nvars && CONSP (varlist); argnum++)
+  for (argnum = 0; CONSP (varlist); argnum++)
     {
       maybe_quit ();
       elt = XCAR (varlist);
@@ -1017,12 +1016,11 @@ DEFUN ("let", Flet, Slet, 1, UNEVALLED, 0,
       else
        temps[argnum] = eval_sub (Fcar (Fcdr (elt)));
     }
-  nvars = argnum;
 
   lexenv = Vinternal_interpreter_environment;
 
   varlist = XCAR (args);
-  for (argnum = 0; argnum < nvars && CONSP (varlist); argnum++)
+  for (argnum = 0; CONSP (varlist); argnum++)
     {
       Lisp_Object var;
 
diff --git a/test/src/eval-tests.el b/test/src/eval-tests.el
index b2b7dfefda..85dc2a53ae 100644
--- a/test/src/eval-tests.el
+++ b/test/src/eval-tests.el
@@ -226,4 +226,37 @@ eval-tests/backtrace-in-batch-mode/demoted-errors
       (should (equal (string-trim (buffer-string))
                      "Error: (error \"Boo\")")))))
 
+(ert-deftest eval-tests/let ()
+  (should (equal (eval '(let (a)
+                          a)
+                       t)
+                 nil))
+
+  (should (equal (eval '(let (a b)
+                          (list a b))
+                       t)
+                 '(nil nil)))
+
+  (should (equal (eval '(let ((a 1))
+                          a)
+                       t)
+                 1))
+
+  (should (equal (eval '(let ((a 1) b)
+                          (list a b))
+                       t)
+                 '(1 nil)))
+
+  ;; (error "`let' bindings can have only one value-form" a 1 2)
+  (should-error (eval '(let ((a 1 2))
+                         a)
+                      t)
+                :type 'error)
+
+  ;; (wrong-type-argument symbolp (a))
+  (should-error (eval '(let (((a) 1))
+                         a)
+                      t)
+                :type 'wrong-type-argument))
+
 ;;; eval-tests.el ends here
-- 
2.30.1