Re: The Paul Graham's "Revenge of the nerds" cummulator function and the solution in Emacs Lisp

[Tassilo Horn <tassilo@member.fsf.org>]

Tiago Charters de Azevedo <tca@cii.fc.ul.pt> writes:

Hi Tiago,

> Should not the function foo in Common Lisp work with Emacs Lisp?
>
> (defun foo (n)
>   (lambda (i)     
>     (incf n i))) 

No, that won't work because emacs has no lexical scoping and that code
is a closure.  See the elisp manual:

,----[ (info "(elisp)Extent") ]
|    To illustrate this, the function below, `make-add', returns a
| function that purports to add N to its own argument M.  This would work
| in Common Lisp, but it does not do the job in Emacs Lisp, because after
| the call to `make-add' exits, the variable `n' is no longer bound to
| the actual argument 2.
| 
|      (defun make-add (n)
|          (function (lambda (m) (+ n m))))  ; Return a function.
|           => make-add
|      (fset 'add2 (make-add 2))  ; Define function `add2'
|                                 ;   with `(make-add 2)'.
|           => (lambda (m) (+ n m))
|      (add2 4)                   ; Try to add 2 to 4.
|      error--> Symbol's value as variable is void: n
| 
|    Some Lisp dialects have "closures," objects that are like functions
| but record additional variable bindings.  Emacs Lisp does not have
| closures.
`----

Bye,
Tassilo
-- 
Chuck Norris once rode a bull, and nine months later it had a calf.