From: Conrad Schneiker <schneik@...>
Date: 2000-09-18T15:10:02+09:00
Subject: [ruby-talk:4983] Re: Perl 6 rumblings -- RFC 225 (v1) Data:Superpositions

Hi,

Charles Hixson wrote:
....
> This is a nice series of operators.
> any?  == "there exists"
> all? == "all"
> I think it's missing a "there exists a unique x such that"

Since "for every given x" ... "such that" is implied/presumed in your
first 2 cases, how about something like:

any?  == "for every given x, there exists at least 1 x such that"
all? == "for every given x, such that"
unique? == "for every given x, there exists only 1 x such that"
only_one? == "for every given x, there exists only 1 x such that"

'unique?' seems slightly ambiguous, since it might be read as each
instance being unique. (But maybe this is just due to having used "sort
| uniq -<various params>" in the past, or the distinction between sets
with unique elements, and bags with possibly repeated elements.)
 
> However, if one starts getting into lazy evaluation, please remember that the
> second order propositional calculous is undecideable.  Or am I assuming to
> broad an applicability for these operators?

Well, it doesn't hurt to plan ahead for maximum power. However, I don't
happen to know what the relevant connection is between lazy evaluation
and the undecidability of the 2nd order PC. (You meant predicate
calculus, not propositional calculus, right?)

-- 
Conrad Schneiker
(This note is unofficial and subject to improvement without notice.)