Use "if" as a name of a method.

func_if.rb

#!/usr/bin/ruby1.9.1 -Ku
# -*- mode: ruby; coding: utf-8-unix -*-

class Foo
   def if(p,t,f)
      if p.call then
         t.call
      else
         f.call
      end
   end

   def hoge
      self.if(lambda{ 1 != 2 },
              lambda{ puts "True!" },
              lambda{ puts "False!"})
   end
end

Foo.new.hoge

# Local Variables:
# ruby-indent-level: 3
# indent-tabs-mode: nil
# End: