Change the binding of a Proc in Ruby

Question

I have this code: ``` l = lambda { a } def some_function a = 1 end ``` I just want to access `a` by the lambda and a special scope which has defined `a` already somewhere like inside `some_function` in the example, or just soon later in the same scope as: ``` l = lambda { a } a = 1 l.call ``` Then I found when calling `l`, it is still using its own binding but not the new one where it was called. And then I tried to use it as: ``` l.instance_eval do a = 1 call end ``` But this also failed, it is strange that I can't explain why. I know the one of the solution is using `eval`, in which I could special a binding and executing some code in text, but I really do not want to use as so. And, I know it is able to use a global variable or instance variable. However, actually my code is in a deeper embedded environment, so I don't want to break the completed parts if not quite necessary. I have referred the `Proc` class in the documentation, and I found a function names `binding` that referred…

Accepted Answer

You can try the following hack:

```
class Proc
  def call_with_vars(vars, *args)
    Struct.new(*vars.keys).new(*vars.values).instance_exec(*args, &self)
  end
end
```

To be used like this:

```
irb(main):001:0* lambda { foo }.call_with_vars(:foo => 3)
=> 3
irb(main):002:0> lambda { |a| foo + a }.call_with_vars({:foo => 3}, 1)
=> 4
```

This is not a very general solution, though. It would be better if we could give it `Binding` instance instead of a Hash and do the following:

```
l = lambda { |a| foo + a }
foo = 3
l.call_with_binding(binding, 1)  # => 4
```

Using the following, more complex hack, this exact behaviour can be achieved:

```
class LookupStack
  def initialize(bindings = [])
    @bindings = bindings
  end

def method_missing(m, *args)
    @bindings.reverse_each do |bind|
      begin
        method = eval("method(%s)" % m.inspect, bind)
      rescue NameError
      else
        return method.call(*args)
      end
      begin
        value = eval(m.to_s, bind)
        return value
      rescue NameError
      end
    end
    raise NoMethodError
  end

def push_binding(bind)
    @bindings.push bind
  end

def push_instance(obj)
    @bindings.push obj.instance_eval { binding }
  end

def push_hash(vars)
    push_instance Struct.new(*vars.keys).new(*vars.values)
  end

def run_proc(p, *args)
    instance_exec(*args, &p)
  end
end

class Proc
  def call_with_binding(bind, *args)
    LookupStack.new([bind]).run_proc(self, *args)
  end
end
```

Basically we define ourselves a manual name lookup stack and `instance_exec` our proc against it. This is a very flexible mechanism. It not only enables the implementation of `call_with_binding`, it can also be used to build up much more complex lookup chains:

```
l = lambda { |a| local + func(2) + some_method(1) + var + a }

local = 1
def func(x) x end

class Foo < Struct.new(:add)
  def some_method(x) x + add end
end

stack = LookupStack.new
stack.push_binding(binding)
stack.push_instance(Foo.new(2))
stack.push_hash(:var => 4)

p stack.run_proc(l, 5)
```

This prints 15, as expected :)

UPDATE: Code is now also available at Github. I use this for one my projects too now.

Change the binding of a Proc in Ruby

Solution

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