[Openmcl-devel] A plug for nx1-combination-hook

Daniel Weinreb dlw at itasoftware.com
Thu Sep 10 15:46:00 UTC 2009


I'd like to see this added.

Ron Garret wrote:
> 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  
>>> MacRoman).
>>> 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  
> trial balloon.
>
> I have this little hack that allows CCL to support ((...) ...)  
> syntax.  Source code is at:
>
> http://www.flownet.com/ron/lisp/nx1-combination-hook.lisp
>
> 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))))))
> Y
> ? (define fact* (λ (f) (λ (n) (if (zerop n) 1 (* n (f (1- n)))))))
> FACT*
> ? ((y fact*) 15)
> 1307674368000
> ?
>
>
> 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?
>
> rg
>
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