From: Peter Hickman <peterhickman386@...>
Date: 2013-02-12T00:18:57+09:00
Subject: Playing with DSLs, a question or two

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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.

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<div dir=3D"ltr">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 he=
re&#39;s an example of one I am playing with (based on the classic dungeon =
adventure problem)<div>
<br></div><div><div>class Adventure</div><div>=A0 def initialize</div><div>=
=A0 =A0 @rooms =3D Hash.new</div><div>=A0 =A0 @current_room =3D nil</div><d=
iv>=A0 end</div><div><br></div><div>=A0 def room(reference, &amp;block)</di=
v><div>=A0 =A0 if @rooms.has_key?(reference)</div>
<div>=A0 =A0 =A0 puts &quot;Error: Room #{reference} has already been defin=
ed&quot;</div><div>=A0 =A0 else</div><div>=A0 =A0 =A0 @rooms[reference] =3D=
 {:short =3D&gt; &#39;&#39;, :long =3D&gt; &#39;&#39;, :exits =3D&gt; Hash.=
new}</div><div>=A0 =A0 end</div>
<div><br></div><div>=A0 =A0 @current_room =3D reference</div><div><br></div=
><div>=A0 =A0 yield block</div><div>=A0 end</div><div><br></div><div>=A0 de=
f short(text)</div><div>=A0 =A0 @rooms[@current_room][:short] =3D text</div=
><div>=A0 end</div>
<div><br></div><div>=A0 def long(text)</div><div>=A0 =A0 @rooms[@current_ro=
om][:long] =3D text</div><div>=A0 end</div><div><br></div><div>=A0 def door=
(direction, destination, &amp;block)</div><div>=A0 =A0 if @rooms[@current_r=
oom][:exits].has_key?(direction)</div>
<div>=A0 =A0 =A0 puts &quot;Error: Room #{@current_room} already has an exi=
t defined in #{direction}&quot;</div><div>=A0 =A0 else</div><div>=A0 =A0 =
=A0 @rooms[@current_room][:exits][direction] =3D {:where =3D&gt; destinatio=
n, :condition =3D&gt; block_given? ? block : true}</div>
<div>=A0 =A0 end</div><div>=A0 end</div><div>end</div><div><br></div><div>c=
lass Object</div><div>=A0 def adventure(&amp;block)</div><div>=A0 =A0 a =3D=
 Adventure.new</div><div><br></div><div>=A0 =A0 a.instance_eval(&amp;block)=
</div><div>=A0 end</div>
<div>end</div><div><br></div><div>adventure do</div><div>=A0 room :cave do<=
/div><div>=A0 =A0 short &quot;You are in a large cave&quot;</div><div>=A0 =
=A0 long &quot;You are in a very large cave&quot;</div><div>=A0 =A0 door :w=
est, :another_cave</div>
<div>=A0 =A0 door :east, :exit do</div><div>=A0 =A0 =A0 if player.has_item(=
:key)</div><div>=A0 =A0 =A0 =A0 true</div><div>=A0 =A0 =A0 else</div><div>=
=A0 =A0 =A0 =A0 false</div><div>=A0 =A0 =A0 end</div><div>=A0 =A0 end</div>=
<div>=A0 end</div><div><br></div><div>=A0 room :another_cave do</div>
<div>=A0 =A0 short &quot;A cave&quot;</div><div>=A0 =A0 long &quot;Oh great=
 ... another cave&quot;</div><div>=A0 =A0 door :east, :cave</div><div>=A0 e=
nd</div><div><br></div><div>=A0 room :exit do</div><div>=A0 =A0 short &quot=
;Freedom&quot;</div>
<div>=A0 =A0 long &quot;Freeeeeeeeeeedom!!!!!!&quot;</div><div>=A0 =A0 door=
 :west, :cave</div><div>=A0 end</div><div><br></div><div>=A0 require &#39;p=
p&#39;</div><div>=A0 pp @rooms</div><div>end</div></div><div><br></div><div=
 style>Now this works just fine but a couple of things just don&#39;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=A0variable=A0to help tie things together. Is there some better way o=
f doing this? Perhaps by passing it as a parameter somehow.</div>
<div style><br></div><div style>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=
?</div><div style><br></div><div style>Also how do I stop, for example, the=
 door method being called outside of the room method? I have a solution whe=
re I set @current_room to nil after the yield and then each method checks t=
o see if it is set and if not it errors. But again this seems to be quite a=
 pain.</div>
<div style><br></div></div>

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