From: Dave Burt <dave@...>
Date: 2006-05-26T16:09:23+09:00
Subject: Re: Method '!='

Victor Shepelev wrote:
> > > > > So, other question: is there way for uniform compare objects by some
> > > > > operator? (where operator is either == or != or <, or even include?)
> > > The point was to do things like this:
> > >
> > > def filter(object, op, criteria)
> > >   object.send op, criteria
> > > end
> > >
> > > filter 'a', :==, 'b'
> > > filter 'Ruby', :include?, 'Ru'
> > > filter 110, :<, 15
> > >
> > > #filter 'a', :!=, 'b' <== doesn't work! :(
>
> OK, some more pieces of puzzle :)
>
> 1. really, the former is used as bunch of filters:
>
> Request.filter [
>   [:base_type, :==, 'cpu'],
>   [:fullname, :include?, 'Intel'],
>   [:frequency, :>, 2300]
> ]
>
> 2. I'm planning inside Request to do the following:
> a) search some pre-fetched array (through Array#select)
> b) so some additional request to database
>
> For (a), filter as lambda is good; but for (b), I need some format I can
> convert into plain SQL.

From ZenSpider's Ruby QuickRef, with a couple of corrections:
http://www.zenspider.com/Languages/Ruby/QuickRef.html#22

Operators by Precedence:
:: .
[]
**
-(unary) +(unary) ! ~
*  /  %
+  -
<<  >>
&
|  ^
>  >=  <  <=
<=> == === != =~ !~
&&
||
.. ...
=(+=, -=...)
not
and or

All of the above are just methods except these:

=, .., ..., !, not, &&, and, ||, or, !=, !~, ::

In addition, assignment operators(+= etc.) are not user-definable.

You may want to transform &&, || and != into a certain proc and a
certain other string for use in SQL.

Perhaps something like this:

op = Hash.new do |h, k|
  { :sql => k,
    :proc => proc {|obj, *args| obj.send(k, *args) } }
end

op["&&"] = {:sql => " AND ", :proc => proc {|l,r| l && r } }
op["||"] = {:sql => " OR ", :proc => proc {|l,r| l || r } }
op["!="] = {:sql => " <> ", :proc => proc {|l,r| l != r } }
op["=="] = {:sql => " = ", :proc => proc {|l,r| l == r } }

op[">"][:sql]  #=> ">"
op[">"][:proc].call(2, 1)  #=> true

Cheers,
Dave