Extracting instance variables from a block?

Hello fellow Rubyists,

I am writing a tiny custom web framework, similar in nature to web.py
(http://webpy.org/). However, I’d like someone with better
metaprogramming-foo to help me clean up some syntax a bit.

Here’s a very basic abbreviated version of the framework:

class Web
def initialize
@pages = {}
@vars = {}
end

Associates a URI regular expression with a block of code.

def page(regexp, &action)
@pages[/^#{regexp}$/] = action
end

Sets a variable for use in the template. See the example below.

def []=(var, val)
@vars[var] = val
end

Renders the given ERB template file.

def render(template)
o = Object.new
def o.b; binding; end
@vars.each { |var, val| o.instance_variable_set("@#{var}", val) }
print CGI.new.header(‘text/html’)
ERB.new(File.read(template)).run(o.b)
end

Compares each regexp in @pages to ENV[‘REQUEST_URI’]. If a match

is found, calls the associated action block, passing in the match

results as arguments.

def run
for regexp, action in @pages
if ENV[‘REQUEST_URI’] =~ regexp
action[*$~.to_a[1…-1]]
return
end
end
# Oh noes! 404 error handling stuff goes here.
end
end

A simple application would look something like this:

web = Web.new

web.page ‘/hello/(\w+)’ do |name|
web[:title] = ‘Hello’
web[:name] = name
web.render ‘hello.rhtml’
end

web.run

Contents of hello.rhtml

<%= @title %>

Hello, <%= @name %>!

How are you today?

The Web#run method reads the request URI, and sees if it matches with
any of your defined regexps. If it does, it runs the block of code you
associated that URL with (passing in any needed parameters). So when you
visit http://example.com/hello/_why, it displays a nice welcome message
to _why.

As you can see, you can use web[:foo] = ‘bar’ to set an instance
variable, @foo, in the template’s binding. This is all nice and great,
but the syntax is a bit… crusty. I’d love to do something like this
instead:

web.page ‘/hello/(\w+)’ do |name|
@title = ‘Hello’
@name = name
web.render ‘hello.rhtml’
end

So, my question: is there any way to capture the instance variables set
in the block, and use them for a template binding elsewhere? Or at least
do something else that gives me my desired results?

Thanks,
Chris

On Aug 7, 2006, at 6:08 PM, Chris Eskow wrote:

Hello fellow Rubyists,

I am writing a tiny custom web framework, similar in nature to web.py
(http://webpy.org/). However, I’d like someone with better
metaprogramming-foo to help me clean up some syntax a bit.

Here’s a very basic abbreviated version of the framework:

Posted via http://www.ruby-forum.com/.

Hey Chris-

Cool little class ;) Take a look at this, you should be able to use

something like it to do what you want.

class Web
def page(url, &blk)
@url = url
instance_eval &blk
end
end

web = Web.new

web.page ‘/hello/(\w+)’ do |name|
@title = ‘Hello’
@name = “Ernie”
end

puts web.inspect

=> #<Web:0x32c998 @title=“Hello”, @name=“Ernie”, @url="/hello/(\w+)">

-Ezra

Ezra Z. wrote:

[snip]

class Web
def page(url, &blk)
@url = url
instance_eval &blk
end
end

web = Web.new

web.page ‘/hello/(\w+)’ do |name|
@title = ‘Hello’
@name = “Ernie”
end

puts web.inspect

=> #<Web:0x32c998 @title=“Hello”, @name=“Ernie”, @url="/hello/(\w+)">

-Ezra

Thanks for the suggestion (and the compliment), but the problem with
that is that for every page you define, the block of code you associate
it with will be called. So, if you define several pages like so:

web.page(’/’) { web.render ‘home.rhtml’ }
web.page(’/about’) { web.render ‘about.rhtml’ }
web.page(’/hello/(\w+)’) { |name| @name = name; web.render ‘hello.rhtml’
}

It will render all three templates at once! But it did get me
thinking… What if I instance_eval it within Web#run? For example:

class Web
def initialize
#…
@obj = Object.new
def @obj.b; binding; end
end

def render(template)
#…
ERB.new(File.read(template)).run(@obj.b)
end

def run
#…
@obj.instance_eval(&action)
end
end

That sort of works nice… But you can’t pass in the URL parameters (the
ones that your URL regexps match). Is there some way that you can call a
Proc object (with parameters), but within the context of a specific
object’s binding?

Chris

Wait! I got it!

Instead of keeping an object to store the binding in, I can store the
Proc object’s binding itself:

class Web
def initialize
@pages = {}
@binding = nil
end

def page(regexp, &action)
@pages[/^#{regexp}$/] = action
end

def render(template)
print CGI.new.header(‘text/html’)
ERB.new(File.read(template)).run(@binding)
end

def run
for regexp, action in @pages
if ENV[‘REQUEST_URI’] =~ regexp
@binding = action.binding
action[*$~.to_a[1…-1]]
return
end
end
end
end

As you can see, once I found the matching page, I store its action’s
binding (by calling Proc#binding) into an instance variable (@binding),
right before I actually call the action. Then, when running the ERB
template, I use @binding for template’s binding.

Thanks for your help, anyway!

Chris

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