Dynamically Scoped Variables

Whenever I write complex systems, I find I need a way to keep context information lying around. For example I may pick up a set of user preferences for colors at the top level of some code, but then I need them when I get fifteen levels deep, somewhere within the bowels of a component’s paint method. Or perhaps I get a database connection at the start of request handling, but then need to use it when I get deeply nested inside some application code.

These types of scenario seem to have no easy answer. Sometimes you solve it by passing a common parameter to all your methods. This parameter then contains references to all the context information needed by the application code. But this is a messy approach: it means that all your methods have to accept and pass on a parameter that they don’t necessarily need themselves.

A variant of the above is to pass the context object to every constructor, and then store a reference in an instance variable. This suffers from the same drawbacks; every object is carrying around payload that it might not itself need.

Sometimes you can get away with using singletons to store this kind of stuff, but this rapidly breaks down (or at least becomes unwieldy) in the face of multi-threading.

There is another answer, though: dynamically scoped variables.

Most languages offer lexically-scoped variables. When a program is compiled, variable names are looked up by first examining the enclosing scope, then the scope that lexically encloses that scope, and so on. Variables are bound according to their static location in the source code.

However, another kind of variable binding is remarkably useful for passing around context information. Dynamically scoped variables are resolved not at compile time but at run time. When a dynamically scoped variable is referenced, the runtime looks for an appropriate variable in the current stack frame. If none is found, it looks in the caller’s stack frame, and then in that stack frame’s caller, and so on. That way you can set the context in one method, then call multiple levels deep, and still reference it.

Many languages offer dynamically scoped variables: Lisp, TCL, Postscript, and Perl to name a few. In Perl, you could use local to achieve the effect:

  sub update_widget() {
      print "<$color>$name</$color>\n";
  }

  sub update_screen() {
      update_widget;
  }

  sub do_draw() {
      local $name = "dave";
      local $color = "red";
      update_screen();
  }

Although easy to use, locals in Perl are hard to control. And Perl’s features don’t help me much anyway; I needed a Ruby solution. I came up with something that’ll let me do the following.

  def update_widget
    name = find_in_context(:name)
    color = find_in_context(:color)
    puts "<#{color}>#{name}</#{color}>"
  end

  def update_screen
    update_widget
  end

  with_context(:name => 'dave', :color => 'red') do
    update_screen
  end

The with_context block establishes a set of dynamic variables (the parameters to the call). Within any method called at any level during the execution of the with_context block, a call to find_in_context looks up the appropriate dynamic variable’s value and returns it.

The implementation I came up with allows nested dynamic scopes, so the code:

  with_context(:name => 'dave', :color => 'red') do
    with_context(:name => 'fred', :color => 'green') do
      update_screen
    end
    update_screen
  end

outputs:

  <green>fred</green>
  <red>dave</red>

The actual implementation itself is a tad ugly (and I’d welcome alternatives), but right now I view it as something of a singing pig.

  def with_context(params)
    finder = catch(:context) { yield }
    finder.call(params)  if finder
  end

  def find_in_context(name)
    callcc do |again|
      throw(:context, proc {|params|
        if params.has_key?(name)
          again.call(params[name])
        else
          raise "Can't find context value for #{name}"
        end
      })
    end
  end

Update…

And of course, it took less than eight hours for a more elegant implementation to surface (I love the Ruby community). Tanaka Akira posted:

  def with_context(params)
    Thread.current[:dynamic] ||= []
    Thread.current[:dynamic].push params
    begin
      yield
    ensure
      Thread.current[:dynamic].pop
    end
  end

  def find_in_context(name)
    Thread.current[:dynamic].reverse_each {|params|
      return params[name] if params.has_key? name
    }
    raise "Can't find context value for #{name}"
  end

Update #2…

And Avi Bryant massages the original into this masterpiece of minimalism…

  def with_context(params)
   k, name = catch(:context) {yield; return}
   k.call(params[name] || find_in_context(name))
  end

  def find_in_context(name)
    callcc{|k| throw(:context, [k, name])}
  end