[taos-glug] Can "first class" procedures have more than one argument?
- From: Philip Ansteth <pansteth@xxxxxxxxxxx>
- To: taos-glug@xxxxxxxxxxxxx
- Date: Sun, 10 Aug 2003 17:32:17 -0600
I haven't found it stated one way or the other in SICP. In Section
1.3.4, all the examples have one argument, i.e., every instance of
"(lambda (x) . . ." has
(x) only, not (x y) nor (x y z).
Is it necessary to have only one?
Here's why I'm asking.
When I got to Exercise 1.44, I thought the "small number", dx,
should be changeable in order to control the smoothing process.
(I guessed, smaller dx, smoother curve). Also, I thought the
idea of a global variable was not so great.
So, I tried to
write 'smooth' first as
(define (smooth f dx) (lambda (x) . . .
and later as
(define (smooth f) (lambda (x dx) . . .
They both yield the same result;
guile> (define (smooth f dx)
(lambda (x)
(/ (+ (f (- x dx)) (f x) (f (+ x dx))) 3)))
guile> ((smooth cos .01) 1.0)
0.540284295941361
guile> (define (smooth f)
(lambda (x dx)
(/ (+ (f (- x dx)) (f x) (f (+ x dx))) 3)))
guile> ((smooth cos) 1.0 .01)
0.540284295941361
But when I tried to combine this with the "repeated" procedure as suggested
in the exercise, I couldn't ever make it work. I thought that, instead of
(((repeated smooth 2) cos) 1.0)
I should be able to write
(((repeated smooth 2) cos .001) 1.0)
or
(((repeated smooth 2) cos) 1.0 .001)
But these didn't seem to work.
So, I looked into the way I wrote 'repeated', which invoked, 'compose'
which I wrote as follows:
(define (compose f g)
(lambda (arg)
(f (g arg))))
I gave up at this point because it looked as if, even if it were
possible to add more arguments to f and g, it would disturb several working
procedures.
My pragmatic tendencies overcame my curiosity.
So I had to return to using (define dx .0001), and dx thus become a global
value. Globals are usually considered to be a BAD THING.
I reviewed the book to see if the authors had made any comment. I didn't
see any in Section 1.3.4. They just say, " . . . along with the
definition '(define dx 0.00001)'. (This is statement is after they
define 'deriv'.)
(I think that, in this case, the authors could be accused of trying
to conceal a defect in their presentation by "hiding it in plain sight."
I'm learning from these guys, but they sure don't make it easy.)
Clearly one could address my complaints by putting smooth, repeated, compose
and dx into a block, which
would remove need for a global variable (although it would be global
to that block). Furthermore, doing so would also hide the potentionally
reusable 'compose' and 'repeated' procedures.
I guess it's also possible to pass a compound object as the sole argument,
and then have the procedure dissect it.
Any thoughts, anybody?
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