From: Peter Hickman <peterhickman386@...>
Date: 2013-02-12T05:42:06+09:00
Subject: Re: Playing with DSLs, a question or two

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Thanks Ryan.

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

Thanks for that.


On 11 February 2013 17:18, Ryan Cook <cookrn@gmail.com> wrote:

> 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 <
> peterhickman386@googlemail.com> wrote:
>
>> 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.
>>
>>
>
>
> --
> Ryan Cook
> 720.319.7660
>

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<div dir=3D"ltr">Thanks Ryan.<div><br></div><div style>That was just the so=
rt of thing I was looking for.</div><div style><br></div><div style>Thanks =
for that.</div></div><div class=3D"gmail_extra"><br><br><div class=3D"gmail=
_quote">
On 11 February 2013 17:18, Ryan Cook <span dir=3D"ltr">&lt;<a href=3D"mailt=
o:cookrn@gmail.com" target=3D"_blank">cookrn@gmail.com</a>&gt;</span> wrote=
:<br><blockquote class=3D"gmail_quote" style=3D"margin:0 0 0 .8ex;border-le=
ft:1px #ccc solid;padding-left:1ex">
<div dir=3D"ltr">Hey Peter,<div><br></div><div>How does this look?=A0<a hre=
f=3D"https://gist.github.com/cookrn/4755773" target=3D"_blank">https://gist=
.github.com/cookrn/4755773</a></div><div><br></div><div>By making the Room =
a separate object, the scoping might be more as you had hoped e.g. the door=
 method</div>

<div><br></div><div>Regarding the yield vs. instance_eval -- this is a matt=
er of opinion. If you wanted to be very explicit, you could yield the adven=
ture and room objects to the blocks given to those methods. This makes your=
 DSL seem a bit less magic, but maybe that&#39;s ok.</div>

<div><br></div><div>I gave a talk on DSLs at Boulder Ruby a while back that=
 you might find interesting:=A0<a href=3D"https://speakerdeck.com/cookrn/ds=
ls-as-teaching-tools" target=3D"_blank">https://speakerdeck.com/cookrn/dsls=
-as-teaching-tools</a></div>

<div><br></div><div>Ryan</div></div><div class=3D"gmail_extra"><div><div cl=
ass=3D"h5"><br><br><div class=3D"gmail_quote">On Mon, Feb 11, 2013 at 8:18 =
AM, Peter Hickman <span dir=3D"ltr">&lt;<a href=3D"mailto:peterhickman386@g=
ooglemail.com" target=3D"_blank">peterhickman386@googlemail.com</a>&gt;</sp=
an> wrote:<br>

<blockquote class=3D"gmail_quote" style=3D"margin:0 0 0 .8ex;border-left:1p=
x #ccc solid;padding-left:1ex"><div dir=3D"ltr">I am playing around with wr=
iting some DSLs to help me get more familier with how they work and I have =
a couple of questions. First here&#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=
>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 refer=
enced in short, long and door. This seems clunky - it is basically a global=
=A0variable=A0to help tie things together. Is there some better way of doin=
g this? Perhaps by passing it as a parameter somehow.</div>


<div><br></div><div>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>=
<br></div><div>Also how do I stop, for example, the door method being calle=
d outside of the room method? I have a solution where I set @current_room t=
o nil after the yield and then each method checks to see if it is set and i=
f not it errors. But again this seems to be quite a pain.</div>


<div><br></div></div>
</blockquote></div><br><br clear=3D"all"><div><br></div></div></div><span c=
lass=3D"HOEnZb"><font color=3D"#888888">-- <br>Ryan Cook<br><a href=3D"tel:=
720.319.7660" value=3D"+17203197660" target=3D"_blank">720.319.7660</a>
</font></span></div>
</blockquote></div><br></div>

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