Professional Ruby on Rails™

14.1. Eval and Bindings

Many scripting languages have a facility to allow an arbitrary string to be executed at runtime. In Ruby, that feature comes from the eval method. Here's an example of that method:

>> eval("2 + 2")
=> 4

By default, variables in the code string are evaluated in the current context, as follows:

>> x = 3
=> 3
>> y = 5
=> 5
>> eval("x * y")
=> 15

However, you can evaluate the string in a more-or-less arbitrary environment by passing in an optional second argument, which must be a Ruby binding. Bindings are kind of odd entities that encapsulate the context of a block, which is basically a lookup table that stores the values of all accessible variables and constants. From the binding, the state of variable values when the block is created can be revisited.

Every block or Proc object defines its own binding, within which it can set local variable values. Under normal circumstances, this binding is accessed only when the block is executed, to determine the values of variables used by the block. However, you can retrieve just the binding and execute an arbitrary statement within it using the variable values as defined when the block was created.

Here's a test script that shows how bindings work:

def some_crazy_thing
  a = 10
  b = 20
  Proc.new {c = 10}
end

p eval("a * b", some_crazy_thing.binding)

The method some_crazy_thing sets a couple of variables and returns a Proc object. The last line of the script takes that block, grabs its binding, and tries to add two variables. ...