From: "Aleksi Niemelä" <aleksi.niemela@...>
Date: 2000-11-09T04:23:27+09:00
Subject: [ruby-talk:6175] Lazy operators

> Finally, I'd like to mention something that I have sometimes wished
> for - referring to previously mentioned variables or cached results.
> This is probably the ultimate in lazy programming, I don't know,
> but how many times have you wished you could stop repeating
> yourself in the following cases?
> 
> if a == "up" || a== "down" || a == "around" then .....
> 
> Why not:    if a == "up" ^or^ "down" ^or^ "around" .......
> 
> if month > 2 && month < 7 || month == 11 then .....
> 
> Why not: if month > 2 ^and^ < 7 ^or^ == 11 then .....

Oh, this must be your lucky day. I just noticed my version of Ruby (which
has started to show some signs of hackery) include Lazy operators. :)

That means you can write the above code as

  if _(month, _ > 2, And < 7, Or == 11 )

Here you have too.

	- Aleksi


module Kernel
  class LazyComparator
    attr_accessor :op, :cmp, :value
    def initialize(op, cmp, value)
      @op, @cmp, @value = op, cmp, value
    end
    def evaluate(truth, var)
      truth2 = var.method(@cmp).call(@value)
      case @op
      when "and"
	return( truth and truth2)
      when "or"
	return(truth or truth2)
      end
    end
  end
  
  class LazyOperator
    def initialize(op)
      @op = op
    end
    def < other
      LazyComparator.new(@op, "<", other)
    end
    def > other
      LazyComparator.new(@op, '>', other)
    end
    def == other
      LazyComparator.new(@op, '==', other)
    end
  end
  
  def _(*ops)
    return And if ops.size == 0
    var = ops.shift
    truth = true
    ops.each { |op| truth = op.evaluate(truth, var) }
    truth
  end
  _ = LazyOperator.new("and")
  And = LazyOperator.new("and")
  Or  = LazyOperator.new("or")
end

for month in 1...12
  printf("%2d", month)
  if _(month, _ > 2, And < 7, Or == 11 ) 
    puts " true"
  else
    puts " false"
  end
end

puts

for a in ["around", "inside out"]
  if _(a, _ =="up", Or == "down", Or == "around")
    puts a
  else
    puts "no " + a
  end
end

# Let me add that the last is easier to write

for a in ["around", "inside out"]
  if %w(up down around).include? a
    puts a
  else
    puts "no " + a
  end
end



Outputting:
 1 false
 2 false
 3 true
 4 true
 5 true
 6 true
 7 false
 8 false
 9 false
10 false
11 true

around
no inside out
around
no inside out