Re: [Emacs-diffs] trunk r113503: * eval.c (Fprogn): Check that BODY is a proper list.

[Paul Eggert]

On 07/23/2013 02:12 PM, Stefan Monnier wrote:
> I don't even think it's a good change: it's good for the compiler to
> have such checks and signal errors, but the interpreter should mostly
> aim to be fast for correct code and safe for all code
Ah, sorry, I thought the Emacs Lisp interpreter was supposed to check
for errors.  I undid the change in trunk bzr 113521.

I also added the following text to the Elisp manual to try to document
the situation.  If this doesn't capture the intent please let me know.

----

  If an expression's first symbol is that of a special form, the
expression should follow the rules of that special form; otherwise,
Emacs's behavior is not well-defined (though it will not crash).  For
example, @code{((lambda (x) x . 3) 4)} contains a subexpression that
begins with @code{lambda} but is not a well-formed @code{lambda}
expression, so Emacs may signal an error, or may return 3 or 4 or
@code{nil}, or may behave in other ways.