From: Brian Candler <b.candler@...>
Date: 2009-08-18T19:53:17+09:00
Subject: Re: How can I extract variables from a ruby snippet

Laran Evans wrote:
> So, using the snippet above, I would learn that there is:
> 
> - a "post" variable which is a hash, expected to have the keys
> "category_id", "title", "body", "extended_body" and "private".
> - a "notify" variable which is an array (indicated by the fact that
> it's iterated over with the .each) of simple values (i.e. it's not a
> multi-dimensional array, or an array of hashes)

You can't tell that - lots of objects implement an 'each' iterator, and 
there isn't really such thing as a 'simple value' in Ruby anyway. The 
number 1 and the Array [1] are both first-class Objects. e.g.

>> h = {1=>"one", 2=>"two"}
=> {1=>"one", 2=>"two"}
>> h.each { |elem| p elem }
[1, "one"]
[2, "two"]

The full solution to what you want is probably to compile the ERB 
template into Ruby, and then compile the Ruby into an sexp (e.g. using 
ParseTree or ripper), and then analyse the sexp. This is quite a lot of 
work.

Otherwise, you could execute the template using erb, and use 
method_missing to pick up attempts to access local variables. From these 
return a dummy object, which again uses method_missing to see what 
methods have been called on that dummy object. If the method_missing 
passes a block then you will need to yield the block with dummy 
arguments.

This may be good enough for what you need, but isn't guaranteed to catch 
all possible execution paths. e.g. what about

  <% if foo %>
    <%= foo.xxx %>
  <% else %>
    <%= bar.yyy %>
  <% end %>
?

You are going to find it hard to exercise both branches of this 
conditional. Similarly with loops, where the nth iteration may behave 
differently to the previous ones.

Rails approaches the problem differently: whenever you render a template 
you pass in a hash of 'local_assigns', and it compiles the template 
assuming that these are the variables which will be substituted. It then 
makes a method signature using this set of variables. If the template is 
rendered again and 'local_assigns' contains a different set of variables 
to substitute, then it is recompiled. The assumption is that normally 
the template will be rendered multiple times using the same set of 
local_assigns variables (just with different values, of course)

See actionpack/lib/action_view/renderable.rb
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