Re: e and pi

[Uday S Reddy]

On 9/18/2010 8:10 PM, Chong Yidong wrote:

> You came up with this
> example:
>
>    (defun make-inc (e)
>      (lambda (m) (+ e m)))
>
>    (let ((f (make-inc 3)))
>      (funcall f 5))
>
> I agree that static scoping is much better in this example.  But we
> could fix this simply by imposing static scope on function arguments.
> So, is there an equivalent scenario in which the proposed change to
> `let' is clearly desireable?

Consider this:

(defvar funny nil)
(let ((e 25))
   (setq funny (lambda (m) (+ e m))))

Now, (funcall funny 2) should return 27.  Instead, with dynamic binding, it 
returns 4.7182...

The point is not whether `e' is being bound in a let or a defun.  Rather, the 
point is that we should be able to use a variable like `e' as a free variable 
in a closure.  We can do so only if it is governed by lexical binding.

Cheers,
Uday