Re: spec 3.1.2.1.2 and the lambda exception

From: Pascal Bourguignon (spam_at_thalassa.informatimago.com)
Date: 08/24/04 

Date: 24 Aug 2004 00:31:51 +0200

Ross Lippert <lippert@MATHSTATION031.MIT.EDU> writes:

> I have been trying to learn some lisp. I have had some experience in
> scheme, which is probably why I even got disturbed by that which is
> the source of my question.
>
> Playing around with the age old bank-account example I did something
> equivalent to
> (defun make-adder (x) (lambda (y) (+ x y)))
> ((make-adder 1) 1)
> and of course, I got an error. What I mean to do here is done by
> using funcall. This is part of that 1-namespace vs 2-namespace
> issue, which divides scheme and lisp, I know.
> [...]
> I guess that 3.1.2.1.2 allows lambdas and no other expressions
> because either:
> 1) there is some good reason to allow lambda expressions in the car.
> 2) there is some good reason to disallow arbitrary function-returning
> expressions in the car.
>
> Which is it and why?

Both.

1: Because a list of the form (lambda (arg...) exp...) IS a function.
   I mean, this is a function:

    +-----------------------------------------------+
    | |
    | +---+---+ +---+---+ +---+---+ +---+---+ |
    | | * | * |-->| * | * |-->| * | * |-->| * |NIL| |
    | +---+---+ +---+---+ +---+---+ +---+---+ |
    | | | | | |
    | v | v v |
    | +--------+ | +------+ +------+ |
    | | LAMBDA | | | EXP0 | | EXP1 | |
    | +--------+ | +------+ +------+ |
    | v |
    | +---+---+ +---+---+ |
    | | * | * |-->| * |NIL| |
    | +---+---+ +---+---+ |
    | | | |
    | v v |
    | +------+ +------+ |
    | | ARG0 | | ARG1 | |
    | +------+ +------+ |
    | |
    +-----------------------------------------------+

    
2: Because there is no difference between a function call and a
   variable evaluatoin. Both are executed with EVAL, and since
   Common-Lisp is a lisp-2, it won't evaluate the first item in a
   sexp.

Note however that you can still play some tricks:

CL-USER> (defun make-adder (x) `(lambda (y) (+ ,x y)))
MAKE-ADDER
CL-USER> (#.(make-adder 1) 1)
2

Of course, the adder will be created at read time instead of run time. 

-- 
__Pascal Bourguignon__                     http://www.informatimago.com/
Our enemies are innovative and resourceful, and so are we. They never
stop thinking about new ways to harm our country and our people, and
neither do we.