From: "Ara.T.Howard" <ahoward@...>
Date: 2004-02-20T20:20:51+09:00
Subject: Re: proposal: let kind_of take more arguments

On Thu, 19 Feb 2004, Simon Strandgaard wrote:

> Date: Thu, 19 Feb 2004 16:29:42 +0100
> From: Simon Strandgaard <neoneye@adslhome.dk>
> Newsgroups: comp.lang.ruby
> Subject: Re: proposal: let kind_of take more arguments
> 
> On Thu, 19 Feb 2004 08:19:33 -0700, Ara.T.Howard wrote:
> > 
> > what do you reccomend (when a case won't do) ?
> > 
> > ...
> >   unless choice.kind_of?(Zero) or choice.kind_of?(One)
> > ...
> > 
> 
> My brain is terrible at dealing with 'unless'.
> If its just "unless something" then its ok.
> But when its "unless a or b and c" then it chokes.
> 
> Am I the only one feeling like this?
> 
> Where can I seek help ?  ;-)

a digital logic book, the simplest, will help.  de morgan's laws..., eg.

noting that 'unless' is the negation of 'if', in otherwords 'unless' => 'if
not'

  let

    ^ => and

    v => or

    ~ => negation 

    Z => kind_of? Zero

    O => kind_of? One 

  then we can say your original statement

    choice.kind_of?(Zero) == false and choice.kind_of?(One) == false

  as simply

    ~Z ^ ~O

  if we want use this conjunction in the negative sense (convert to 'unless')
  we can convert using the following heuristic 'negate each clause and flip
  the operation'

  so

    ~(~Z ^ ~O) => Z v 0

  or

    choice.kind_of?(Zero) or choice.kind_of?(One)


  now we have the _negation_ of the conjuction.  this is want we want though
  since were are trying to use it in the negative sense (unless).  eg. we're
  done:

    unless choice.kind_of?(Zero) or choice.kind_of?(One)
      ...
    end


if you express in simple terms it's pretty easy. eg.

HTH.

-a
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