Re: Another question about lambdas

[Michael Heerdegen <michael_heerdegen@web.de>]

Emanuel Berg <incal@dataswamp.org> writes:

> Are we talking lexical/static `let' inside or outside
> `defun's?
>
> Inside defuns they are very practical, local variables,
> basically, outside defuns they are interesting, rather, I have
> found 2 use cases so far, first, what in other languages are
> refered to as static variables (whose value don't reset
> between calls), second, the possibility to share such
> variables between functions.
>
> This file demonstrate those use cases, other than that I don't
> know what one is supposed to do with them, really, and so far
> they don't do anything global variables don't. So they are
> a method to not have those, basically?

Dunno if this is a good summary, I would be careful.  I find the
question a bit artificial - how is the classification global vs local
functions motivated?  The top-level lexical environment is empty, so
top-level defuns are not interesting as closures.

So we are actually only speaking about the case of functions defined
anywhere besides top-level that are given a global name.  In a
functional language functions with a globally accessible name that have
an alterable state are probably not that sexy - we are instead trying to
make those functions pure, or try to restrict side effects to obvious
well-defined things like creating a window.

Useful cases that come to my mind are more of kind where the state is
never altered but lexical binding eases the definition of bunches of
functions - in a very simple case e.g.

#+begin_src emacs-lisp
(defun my-make-adder (n)
  (lambda (m) (+ m n)))

(defalias 'my-1+ (my-make-adder 1))
(defalias 'my-1- (my-make-adder -1))
#+end_src


Such lexical variables are still different from special top-level
variables (`defvar's): They are private (not accessible "from the
outside").  You can have several of them having the same name.  They
don't clobber a name space.  And no binding created in any context
outside of the lexical context where the functions are defined can ever
change the value of those variables.


Michael.