Re: lambda inside a let or letrec

[pjb@informatimago.com (Pascal J. Bourguignon)]

bolega <gnuist006@gmail.com> writes:

> My apologies in advance to comp.lang.scheme and comp.lang.lisp.
>
> I am trying to run a certain syntax inside emacs lisp.
>
> I know basically how let works
>
> (let (list of pairs of var value) (function))
>
> This is like a lambda function call , only the order is different.
>
> But the novelty i saw reading a book on common lisp or scheme is this
> and i failed to run in emacs. plz tell what modifications are needed
> and i know they are different.
>
> (   (lambda (n) (+ 1 n)) 3)  ;;; works in emacs
>
> (let
> ((a (lambda (n) (+ 1 n))) (b 3)) (a b))   ;;; does NOT work in emacs
>
> basically we are trying to use / abuse the let in that in the pair we
> define a equal to a lambda. Then another pair where a value of b is
> defined.
>
> next, we want a to operate on b.
>
> Why does it fail ?
>
> The scheme/lisp book/paper where it was seen (forgot) used letrec.
>
> Can someone enlighten me how set! and let can be used to formulate
> recursion when the let has no recursion built in it ?

As indicated by David, it's because scheme (and eg. LeLisp) is a
Lisp-1 while emacs lisp and Common Lisp are Lisp-2.



In a Lisp-1 there's a single slot for both values and functions,
functions are just values like others, and a symbol has only one
value, which may be a function or something else.

In this case, both LET and the function application refer the same
unique value of the symbol.

This has the advantage of being simple conceptually, and to be able to
write with a simplier syntax function code of higher order.




In a Lisp-2 there are two slots, one for a value (which may be a
function too) and one for a function (which may only be unbound or a
function).

In this case, depending on the context, the value slot XOR the
function slot is used.  LET binds the value slot, the symbol by itself
evaluates to the contents of its value slots (when it's a special
variable).  On the other hand DEFUN and function application refer the
function slot.   This has the advantage of reducing a lot of collision
problems in writing unhygienic macros, and to offer a double meaning
that matches more closely natural languages.  In a Lisp-2 you can write:
 
   (let ((buffalo (get-a-bull)))
     (flet ((buffalo (what) (do-it what)))
        (buffalo buffalo)))

and have (do-it (get-a-bull)) executed.

On the other hand, if you want to use a value as a function, you have
to go thru APPLY or FUNCALL:

   (let ((fun (lambda (x) (be-happy-with x))))
     (funcall fun 'foot))


In a Lisp-1 you would have to write:

   (let ((buffalo-as-noun (get-a-bull))
         (buffalo-as-verb (lambda (what) (do-it what))))
     (buffalo-as-verb buffalo-as-noon))




Now, IIRC, Pascal Costanza has published in cll a few years ago a
macro that will allow you to write Lisp-1 function applications in a
Lisp-2, so that when you need locally to call a lot of functions
stored in value slots you may write it like in scheme.  
Try to groups.google cll for Pascal Costanza Lisp-1 macro.




-- 
__Pascal Bourguignon__
http://www.informatimago.com