[Openmcl-devel] A plug for nx1-combination-hook
ron at flownet.com
Thu Sep 3 18:57:55 EDT 2009
On Sep 3, 2009, at 3:15 PM, Glen Foy wrote:
> On Sep 3, 2009, at 5:49 PM, Gary Byers wrote:
>> Glen's file seemed to actually be encoded in Macintosh (aka
>> If you mistakenly try to interpret MacRoman (or UTF-8, or lots of
>> other encodings) as ISO-8859-1, you get some of the characters wrong
>> (the ellipsis looks like an acute-accented #\E, etc.) but don't run
>> the risk of getting out-of-synch and confused.
> I've converted the contribs to UTF-8 and removed the offending odd
> chars: an ellipsis char, a copyright char and a bullet char.
> Ron's technique of using the letter lambda as opposed to the word is
> what convinced me. We have to consider the cool factor, here!
As long as Glen brought up the cool factor I'd like to float another
I have this little hack that allows CCL to support ((...) ...)
syntax. Source code is at:
When you load this code you can do Scheme-like things without a ton of
funcalls. In particular, with a suitable definition of λ and DEFINE
(about 15 LOC) you can build a really clean-looking Y-combinator.
Instead of this:
(defun y (f) (funcall (lambda (g) (funcall g g)) (lambda (h) (lambda
(x) (funcall (funcall f (funcall h h)) x)))))
(defun fact* (f) (lambda (n) (if (zerop n) 1 (* n (funcall f (1- n))))))
(funcall (y fact*) 15)
you can now write this:
(define y (λ (f) ((λ (g) (g g)) (λ (h) (λ (x) ((f (h h)) x))))))
(define fact* (λ (f) (λ (n) (if (zerop n) 1 (* n (f (1- n)))))))
((y fact*) 15)
It actually works:
? (define y (λ (f) ((λ (g) (g g)) (λ (h) (λ (x) ((f (h h)) x))))))
? (define fact* (λ (f) (λ (n) (if (zerop n) 1 (* n (f (1- n)))))))
? ((y fact*) 15)
IMHO, being able to actually run the more compact form would be a big
win for pedagogical purposes if nothing else. I think it would also
potentially win some converts who might be on the fence between CL and
Scheme. (And if we can integrate H-M type-checking we might win some
Haskell wannabes as well. :-)
This is not possible in portable CL because the CL standard requires
that ((X ...) ...) to be an error if X is anything other than
'LAMBDA. Note that despite the fact that this allows CCL to produce
behavior that is in technical violation of the standard it would not
actually make CCL a non-conforming implementation. This is because
nx1-combination-hook, as the name implies, doesn't actually *change*
the bahvior of ((...) ...), it merely introduces a hook through which
the user at their option may change the behavior.
So... would anyone object to having nx1-combination-hook added as an
extension to the stock CCL code?
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