Forum: Ruby Playing with DSLs, a question or two

I am playing around with writing some DSLs to help me get more familier
with how they work and I have a couple of questions. First here's an
example of one I am playing with (based on the classic dungeon adventure
problem)

class Adventure
  def initialize
    @rooms = Hash.new
    @current_room = nil
  end

  def room(reference, &block)
    if @rooms.has_key?(reference)
      puts "Error: Room #{reference} has already been defined"
    else
      @rooms[reference] = {:short => '', :long => '', :exits => 
Hash.new}
    end

    @current_room = reference

    yield block
  end

  def short(text)
    @rooms[@current_room][:short] = text
  end

  def long(text)
    @rooms[@current_room][:long] = text
  end

  def door(direction, destination, &block)
    if @rooms[@current_room][:exits].has_key?(direction)
      puts "Error: Room #{@current_room} already has an exit defined in
#{direction}"
    else
      @rooms[@current_room][:exits][direction] = {:where => destination,
:condition => block_given? ? block : true}
    end
  end
end

class Object
  def adventure(&block)
    a = Adventure.new

    a.instance_eval(&block)
  end
end

adventure do
  room :cave do
    short "You are in a large cave"
    long "You are in a very large cave"
    door :west, :another_cave
    door :east, :exit do
      if player.has_item(:key)
        true
      else
        false
      end
    end
  end

  room :another_cave do
    short "A cave"
    long "Oh great ... another cave"
    door :east, :cave
  end

  room :exit do
    short "Freedom"
    long "Freeeeeeeeeeedom!!!!!!"
    door :west, :cave
  end

  require 'pp'
  pp @rooms
end

Now this works just fine but a couple of things just don't seem right.
Notice the @current_room variable that is set in the room method and
referenced in short, long and door. This seems clunky - it is basically 
a
global variable to help tie things together. Is there some better way of
doing this? Perhaps by passing it as a parameter somehow.

Also is the yield at the bottom of the room method the right way to go 
or
should I be calling instance_eval or similar?

Also how do I stop, for example, the door method being called outside of
the room method? I have a solution where I set @current_room to nil 
after
the yield and then each method checks to see if it is set and if not it
errors. But again this seems to be quite a pain.

Hey Peter,

How does this look? https://gist.github.com/cookrn/4755773

By making the Room a separate object, the scoping might be more as you 
had
hoped e.g. the door method

Regarding the yield vs. instance_eval -- this is a matter of opinion. If
you wanted to be very explicit, you could yield the adventure and room
objects to the blocks given to those methods. This makes your DSL seem a
bit less magic, but maybe that's ok.

I gave a talk on DSLs at Boulder Ruby a while back that you might find
interesting: https://speakerdeck.com/cookrn/dsls-as-teaching-tools

Ryan


On Mon, Feb 11, 2013 at 8:18 AM, Peter Hickman <

Thanks Ryan.

That was just the sort of thing I was looking for.

Thanks for that.

Hi,

Dwemthy's Array by Why can also be helpful
http://mislav.uniqpath.com/poignant-guide/dwemthy/

ati

Gulys

Thanks for that. It woud seem that picking an adventure game was the 
best
choice for learning DSLs. At least I understand the domain :)

Peter