Re: <boolean> (was Re: object-class)

[alms@cambridge.apple.com (Andrew LM Shalit)]

At  2:26 PM 11/21/92 +0000, eliot@dcs.qmw.ac.uk wrote:
> 
> Here's the Smalltalk (my DYLAN is not fluent enough yet!):
> 
>     True methods for controlling
> 
>     ifTrue: trueAlternativeBlock ifFalse: falseAlternativeBlock
>         ^trueAlternativeBlock value
> 
> 
>     False methods for controlling
> 
>     ifTrue: trueAlternativeBlock ifFalse: falseAlternativeBlock
>         ^falseAlternativeBlock value
> 
> The above are the implementations of ifTrue:ifFalse: in classes True & False.
> trueAlternativeBlock and falseAlternativeBlock are both closures. ^ is
> return result from method, and value is the message that causes closures to
> evaluate themselves.
> 
> I don't have the DYLAN book to hand so I won't embarrass myself trying to
> translate.  DYLAN equivalents anyone?

(define-generic-function dispatching-if (value true-clause false-clause))

(define-method dispatching-if ((value (singleton #f))
                               (true-clause <function>)
                               (false-clause <function>))
  (false-clause))

(define-method dispatching-if ((value <object>) ;anything besides #f is true
                               (true-clause <function>)
                               (false-clause <function>))
  (true-clause))


If you only want to allow #t to count as 'true' then you would change the
second method definition to specialize on (singleton #t) rather than
<object>.

dispatching-if could be used as follows:

(dispatching-if some-value
                (method () (print "It was true."))
                (method () (print "It was false.")))