instance-eval

I know send takes string or symbol with arguments while instance_eval takes string or block, and their difference could be apparent given receivers. My question is what the ' under the hood ' difference is for the example below? 1234.send 'to_s' # '1234' 1234.instance_eval 'to_s' # '1234' From the fine manual : send(symbol [, args...]) → obj send(string [, args...]) → obj Invokes the method identified by symbol , passing it any arguments specified. [...] When the method is identified by a string, the string is converted to a symbol. and for instance_eval : instance_eval(string [, filename [,

I have the following: class Test @@a = 10 def show_a() puts "a: #{@@a}" end class << self @@b = '40' def show_b puts "b: #{@@b}" end end end Why does following work: Test.instance_eval{show_b} b: 40 => nil But I can't access @@b directly? Test.instance_eval{ @@b } NameError: uninitialized class variable @@b in Object Likewise, the following works t = Test.new t.instance_eval{show_a} a: 10 => nil but the following fails t.instance_eval{ @@a } NameError: uninitialized class variable @@a in Object I don't understand why I can't access the Class Variables directly from the instance_eval blocks. I

I'm working my way through Pickaxe 1.9, and I'm a bit confused by constant-lookup in instance/class_eval blocks. I'm using 1.9.2. It seems that Ruby handles constant-lookup in *_eval blocks the same way it does method-lookup: look for a definition in receiver.singleton_class (plus mixins); then in receiver.singleton_class.superclass (plus mixins); then continue up the eigenchain until you get to #<Class:BasicObject> ; whose superclass is Class; and then up the rest of the ancestor chain (including Object , which stores all the constants you define at the top-level), checking for mixins along

This question already has an answer here: Code block passed to each works with brackets but not with 'do'-'end' (ruby) 3 answers Does anybody know what's causing this error? I'm trying to make a basic rack application. App.rb => class Cherry class << self def app &block Cherry::Application.new &block end end class Application def initialize &block instance_eval &block end def print_start_message puts "Starting server" end def call env [200, {"Content-type" => "text/plain"}, "Hello World"] end end end Config.ru => require 'app' run Cherry.app do print_start_message end EDIT: Apparently I forgot