Ruby: Extended instance variables - ruby

How can I set some instance variables when extending a instance in the same way it can be done when creating it, with initialize.
In this example the variable set when extending is "lost":
module Mod
def self.extended(base)
#from_module = "Hello from Mod"
puts "Setting variable to: #{#from_module}"
end
def hello_from_module
return #from_module
end
end
class Klass
def initialize
#from_class = "Hello from Klass"
end
def hello_from_class
return #from_class
end
end
klass = Klass.new #=> #<Klass:0x00000000ed8618 #from_class="Hello from Klass">
klass.extend(Mod) #=> #<Klass:0x00000000ed8618 #from_class="Hello from Klass">
"Setting variable to: Hello from Mod"
klass.hello_from_class #=> "Hello from Klass"
klass.hello_from_module #=> nil (warning: instance variable #from_module not initialized)

There are a number of ways to do what you describe.
The most common one would be to use instance_variable_get and instance_variable_set:
module ModA
def self.extended(base)
base.instance_variable_set(:#from_module, "Hello from Mod A")
puts "Setting variable to: #{base.instance_variable_get(:#from_module)}"
end
def hello_from_module
return #from_module
end
end
Another common technique is to use any of the eval or exec methods. In this case instance_exec:
module ModB
def self.extended(base)
base.instance_exec { #from_module = "Hello from Mod B" }
puts "Setting variable to: #{base.instance_exec { #from_module }}"
end
def hello_from_module
return #from_module
end
end

Related

How to create a method that executes a previously given block in Ruby?

I have a class that was built for subclassing.
class A
def initialize(name)
end
def some
# to define in subclass
end
end
# usage
p A.new('foo').some
#=> nil
In my use case, I don't want to create a subclass since I need just one instance. Therefore, I'll change the initialize method to support the following usage.
p A.new('foo') { 'YEAH' }.some
#=> YEAH
How could I support the usage above?
BTW: I found the following solutions for a Ruby 1.8.7 project, but they look awkward to me.
class A
def singleton_class
class << self; self; end
end
def initialize(name, &block)
#name = name
self.singleton_class.send(:define_method, :some) { block.call } if block_given?
end
def some
# to define in subclass
end
end
You can store the block argument in an instance variable and call it later on:
class A
def initialize(name, &block)
#name = name
#block = block
end
def some
#block.call
end
end
A.new('foo') { 'YEAH' }.some
#=> "YEAH"
You can also pass arguments into the block:
class A
# ...
def some
#block.call(#name)
end
end
A.new('foo') { |s| s.upcase }.some
#=> "FOO"
Or instance_exec the block in the context of the receiver:
class A
# ...
def some
instance_exec(&#block)
end
end
Which allows you to bypass encapsulation:
A.new('foo') { #name.upcase }.some
#=> "FOO"

Defining new class through method with block

I have a function which defines and returns a new class, with some pre-built methods. E.g.:
def define_class(name, options={}, &block)
klass = Class.new(Class) do
def say_hello
puts "Hello!"
end
def say_goodbye
puts "Adios!"
end
end
parent_class.const_set(form_class, klass)
klass
end
So, for example, this works:
define_class("testing").new.say_hello #=> "Hello!"
But I would like to be able to pass in custom methods through a block, which would then be added to my class, like so:
define_class "testing" do
# ... custom methods
end
Such that this would work:
klass = define_class "testing" do
def interject
puts "Excuse me?"
end
end
klass.new.interject #=> "Excuse me?"
I can't figure out how to make that work though; I've tried instance_eval, class_eval, and yield, and none are producing the desired result.
Try simply:
def define_class(name, options={}, &block)
klass = Class.new(&block)
parent_class.const_set(form_class, klass)
klass
end
If you want to call the block and your own block, you should use class_eval:
def define_class(name, options={}, &block)
klass = Class.new do
def say_hello
puts "Hello!"
end
def say_goodbye
puts "Adios!"
end
class_eval(&block)
end
parent_class.const_set(form_class, klass)
klass
end

How do I using instance variables from within a lambda/Proc defined in a class variable?

I wrote the following code:
class Actions
def initialize
#people = []
#commands = {
"ADD" => ->(name){#people << name },
"REMOVE" => ->(n=0){ puts "Goodbye" },
"OTHER" => ->(n=0){puts "Do Nothing" }
}
end
def run_command(cmd,*param)
#commands[cmd].call param if #commands.key?(cmd)
end
def people
#people
end
end
act = Actions.new
act.run_command('ADD','joe')
act.run_command('ADD','jack')
puts act.people
This works, however, when the #commands hash is a class variable, the code inside the hash doesn't know the #people array.
How can I make the #commands hash be a class variable and still be able to access the specific object instance variables?
You could use instance_exec to supply the appropriate context for the lambdas when you call them, look for the comments to see the changes:
class Actions
# Move the lambdas to a class variable, a COMMANDS constant
# would work just as well and might be more appropriate.
##commands = {
"ADD" => ->(name) { #people << name },
"REMOVE" => ->(n = 0) { puts "Goodbye" },
"OTHER" => ->(n = 0) { puts "Do Nothing" }
}
def initialize
#people = [ ]
end
def run_command(cmd, *param)
# Use instance_exec and blockify the lambdas with '&'
# to call them in the context of 'self'. Change the
# ##commands to COMMANDS if you prefer to use a constant
# for this stuff.
instance_exec(param, &##commands[cmd]) if ##commands.key?(cmd)
end
def people
#people
end
end
EDIT Following #VictorMoroz's and #mu's recommendations:
class Actions
def initialize
#people = []
end
def cmd_add(name)
#people << name
end
def cmd_remove
puts "Goodbye"
end
def cmd_other
puts "Do Nothing"
end
def people
p #people
end
def run_command(cmd, *param)
cmd = 'cmd_' + cmd.to_s.downcase
send(cmd, *param) if respond_to?(cmd)
end
end
act = Actions.new
act.run_command('add', 'joe')
act.run_command(:ADD, 'jill')
act.run_command('ADD', 'jack')
act.run_command('people') # does nothing
act.people
Or
class Actions
ALLOWED_METHODS = %w( add remove other )
def initialize
#people = []
end
def add(name)
#people << name
end
def remove
puts "Goodbye"
end
def other
puts "Do Nothing"
end
def people
p #people
end
def run_command(cmd, *param)
cmd = cmd.to_s.downcase
send(cmd, *param) if ALLOWED_METHODS.include?(cmd)
end
end
act = Actions.new
act.run_command('add', 'joe')
act.run_command(:add, 'jill')
act.run_command('add', 'jack')
act.run_command('people') # does nothing
act.people

class_eval how to pass parameter to method

How do I pass the parameter name in the following case..the name is being is evaluated before being passed to class_eval
class Foo
end
Foo.class_eval %Q{
def hello(name)
p "hello #{name}"
end
}
Sorry about not giving the entire scenario...
I just wanted to add a instance method dynamically to a class and that method should be able to take arguments...
the above code would not compile complaining that the name is not defined as local variable when executing in irb..
Thanks
The other answers are the "right" answer, but you could also just skip interpolating inside the p call:
Foo.class_eval %Q{
def hello(name)
p "hello \#{name}"
end
}
I thought you wanted to change the actual parameter name (possibly useful for completion or when using Pry on dynamic methods), here assuming it's in a global, but could also be passed into a method doing the class_eval:
Foo.class_eval %Q{
def hello(#{$argname})
p "hello \#{$argname}"
end
}
Really simple:
Foo.class_eval do
def hello(name)
p "hello #{name}"
end
end
Try passing a block to class_eval instead of an array (from this link):
class Foo
end
Foo.class_eval {
def hello(name)
p "hello #{name}"
end
}
You then can call the instance method hello in the usual fashion:
boo = Foo.new
boo.hello("you")
which produces:
>> boo.hello("you")
"hello you"
=> nil
class Foo
end
Foo.class_eval do
define_method :hello do |name|
p "hello #{name}"
end
end
Foo.new.hello("coool") # => "hello coool"

Ruby Instance Eval

class Setter
attr_accessor :foo
def initialize
#foo = "It aint easy being cheesy!"
end
def set
self.instance_eval { yield if block_given? }
end
end
options = Setter.new
# Works
options.instance_eval do
p foo
end
# Fails
options.set do
p foo
end
Why does the 'set' method fail?
EDIT
Figured it out...
def set
self.instance_eval { yield if block_given? }
end
Needed to be:
def set(&blk)
instance_eval(&blk)
end
Yep - yield evaluates in the context within which it was defined.
Good write up here, but a simple example shows the problem:
>> foo = "wrong foo"
>> options.set do
?> p foo
>> end
"wrong foo"

Resources