switch.rb

# In response to:
# http://mlomnicki.com/ruby/tricks-and-quirks/2011/02/10/ruby-tricks2.html
# Ruby 1.9.2 has some neat stuff that lets us make a readable
# alternative case statement that calls each method in turn.

# 1.9.2 features used:
#   * hashes are ordered in 1.9.2
#   * cool JSON-style hash syntax
#   * concise lambda syntax

# here's the magic bit of code:

class Object
  def switch( hash )
    hash.each {|method, proc| return proc[] if send method }
    yield if block_given?
  end
end

# The above code allows us to do this:

offer.switch(
  available: -> { order.submit },
  cancelled: -> { order.reject },
  postponed: -> { order.postpone }
) { raise UnknownStateError.new }

# As I see it, the major advantage of this is that we aren't messing
# with any existing methods (Symbol#===), and we don't need an
# is(...).

# Of course, we could make it look more like a control structure by
# defining it on Kernel.  I'm calling this #switch! so we don't
# collide with Object#switch, but you probably wouldn't have both.

module Kernel
  def switch!( thing, hash, &blk )
    thing.switch hash, &blk
  end
end

# Which would let us

switch!( offer,
  available: -> { order.submit },
  cancelled: -> { order.reject },
  postponed: -> { order.postpone }
) { raise UnknownStateError.new }


#-------------------------------------------------------------------

# Here's a variant that doesn't take the default as a block, letting
# us skip the parens:

class Object
  def switch( hash )
    hash.each do |method, proc|
      return proc[] if method == :Default || send( method )
    end
  end
end

offer.switch available: -> { order.submit },
             cancelled: -> { order.reject },
             postponed: -> { order.postpone },
             Default:   -> { raise UnknownStateError.new }

# or:

class Kernel
  def switch!( thing, hash ) thing.switch hash end
end

switch! offer,
  available: -> { order.submit },
  cancelled: -> { order.reject },
  postponed: -> { order.postpone },
  Default:   -> { raise UnknownStateError.new }