Can you eval code in the context of a caller in Ruby?

Question

Essentially I'm wondering if the following can be done in Ruby.

So for example:

def bar(symbol) 
  # magic code goes here, it outputs "a = 100" 
end

def foo
  a = 100 
  bar(:a) 
end

Answer 1

You have to pass foo's context to bar:

def foo
  a = 100
  bar(:a, binding)
end
def bar(sym, b)
  puts "#{sym} is #{eval(sym.to_s, b)}"
end

Answer 2

There is no built-in way to get a callers binding in Ruby in 1.8.X or 1.9.X.

You can use https://github.com/banister/binding_of_caller to work around.

In MRI 2.0 you can use RubyVM::DebugInspector, see: https://github.com/banister/binding_of_caller/blob/master/lib/binding_of_caller/mri2.rb

Working sample in MRI 2.0:

require 'debug_inspector'

def bar(symbol)
  RubyVM::DebugInspector.open do |inspector|
    val = eval(symbol.to_s, inspector.frame_binding(2))
    puts "#{symbol}: #{val}"
  end
end

def foo
  a = 100
  bar(:a)
end

foo
# a: 100

Answer 3

Here's a easier syntax hack, using a passed in block binding:

  def loginfo &block
    what = yield.to_s
    evaled = eval(what, block.binding)
    Rails.logger.info "#{what} = #{evaled.inspect}"
  end

called like this:

  x = 1
  loginfo{ :x }

will log out:

  x = 1

Answer 4

Just FYI, here's a "hacky way". This is my (re-)implementation of well-known ppp.rb:

#!/usr/bin/ruby
#
# better ppp.rb
#

require 'continuation' if RUBY_VERSION >= '1.9.0'

def ppp(*sym)
  cc = nil
  ok = false

  set_trace_func lambda {|event, file, lineno, id, binding, klass|
    if ok
      set_trace_func nil
      cc.call(binding)
    else
      ok = event == "return"
    end
  }
  return unless bb = callcc{|c| cc = c; nil }

  sym.map{|s| v = eval(s.to_s, bb); puts "#{s.inspect} = #{v}"; v }
end

a = 1
s = "hello"
ppp :a, :s

exit 0

This currently fails with 1.9.[012] due to a bug in ruby's set_trace_func.

Answer 5

Check article out Variable Bindings in Ruby

class Reference
  def initialize(var_name, vars)
    @getter = eval "lambda { #{var_name} }", vars
    @setter = eval "lambda { |v| #{var_name} = v }", vars
  end
  def value
    @getter.call
  end
  def value=(new_value)
    @setter.call(new_value)
  end
end

def ref(&block)
  Reference.new(block.call, block.binding)
end

def bar(ref)
  # magic code goes here, it outputs "a = 100" 
  p ref.value
end

def foo
  a = 100 
  bar(ref{:a}) 
end

foo