# Problem with accessors and dsl's

**URL:** <https://www.ruby-forum.com/t/problem-with-accessors-and-dsls/158980>\
**Category:** Ruby\
**Created:** [January 29, 2009, 11:32pm UTC](https://www.ruby-forum.com/t/problem-with-accessors-and-dsls/158980 "2009-01-29T23:32:21Z")\
**Posts on this page:** 3\
**Page:** 1

<div class="post-metadata">

**Author:** ![tsunami](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/t/eb8c5e/32.png) [@tsunami](https://www.ruby-forum.com/u/tsunami)\
**Post date:** [January 29, 2009, 11:32pm UTC](https://www.ruby-forum.com/t/problem-with-accessors-and-dsls/158980/1 "2009-01-29T23:32:21Z")

</div>

I’ve created a class using the following code :

class CommandHandler

```
attr_accessor :command,:arguments

def initialize(command=nil,arguments=nil,what=nil)
    @command = command
    @arguments = arguments
    @what = what
    @code = nil
end

def perform(arguments,what)
    @code.call(arguments,what) if !@code.nil?
end

```

end

and a class named CommandProcessor , which , among other methods has the  
following :

def add\_handler(&block)  
handler = CommandHandler.new  
handler.instance\_eval &block  
@map[handler.command] = handler  
end

so I can add handlers in a DSL-like way . Here is a sample of code in  
action :

add\_handler do  
command=(“get”)  
code=(lambda do |arguments,what|  
begin  
@mech.get(arguments.join(""))  
return @mech.page.body  
rescue  
return “#{arguments} could not be loaded”  
end  
end)  
end  
end

However , by the time the code reaches @map[handler.command] = handler ,  
if I print the handler object , all of it’s attributes appear as nil . I  
specifically added

command=("…")  
and code=("…")

so that the interpreter wouldn’t have to use “heuristics” to deduce what  
I was trying to say . Why isn’t the code working as it should ?

Thanks !

---

<div class="post-metadata">

**Author:** ![tsunami](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/t/eb8c5e/32.png) [@tsunami](https://www.ruby-forum.com/u/tsunami)\
**Post date:** [January 29, 2009, 11:41pm UTC](https://www.ruby-forum.com/t/problem-with-accessors-and-dsls/158980/2 "2009-01-29T23:41:25Z")

</div>

Tsunami S.er wrote:

> so I can add handlers in a DSL-like way . Here is a sample of code in  
> action :
> 
> add\_handler do  
> command=(“get”)

This is interpreted as local variable assignment. You can use the  
following syntaxes instead:

```
 self.command = "get"

```

or (defining #command differently)

```
 command "get"
```

---

<div class="post-metadata">

**Author:** ![tsunami](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/t/eb8c5e/32.png) [@tsunami](https://www.ruby-forum.com/u/tsunami)\
**Post date:** [January 30, 2009, 12:03am UTC](https://www.ruby-forum.com/t/problem-with-accessors-and-dsls/158980/3 "2009-01-30T00:03:34Z")

</div>

Hi –

On Fri, 30 Jan 2009, Tsunami S.er wrote:

> ```
> @code = nil
> 
> ```
> 
> command=(“get”)
> 
> However , by the time the code reaches @map[handler.command] = handler ,  
> if I print the handler object , all of it’s attributes appear as nil . I  
> specifically added
> 
> command=(“…”)  
> and code=(“…”)
> 
> so that the interpreter wouldn’t have to use “heuristics” to deduce what  
> I was trying to say . Why isn’t the code working as it should ?

See Joel’s answer; I just wanted to add the gloss that the interpreter  
doesn’t really use heuristics to figure out what x = y means. It  
_always_ interprets it as an local variable assignment. If it used  
more elaborate heuristics, it might pick up on your non-use of the  
syntactic sugar and decide it was a method call. But it’s very  
literal-minded about that form of expression.

I wouldn’t sweat the “DSL” aspects of it, if by that you mean the  
business of not having to specify a receiver. There’s no real  
advantage to that unless you’re trying to create a full-blown  
self-describing toolset (and even then, I think the “stealth”  
instance\_eval is not as great an idea as it might sometimes seem).

David

–  
David A. Black / Ruby Power and Light, LLC  
Ruby/Rails consulting & training: [http://www.rubypal.com](http://www.rubypal.com)  
Coming in 2009: The Well-Grounded Rubyist ([The Well-Grounded Rubyist](http://manning.com/black2))

[http://www.wishsight.com](http://www.wishsight.com) =\> Independent, social wishlist management!