Understanding instance exec in r

Understanding instance exec in ruby

• 在procs中有上下文

square = lambda { x * x }
x = 20
puts square.call()
# => undefined local variable or method `x' for main:Object (NameError)

• 定义proc时，需要保证里面的变量已经定义

x = 2
square = lambda { x * x }
x = 20
puts square.call()
# => 400

上面得出结果是400，而不是4，是因为proc定义时只是binding了上下文的变量x，并不是x的值。

• 可以是方法

square = lambda { x * x }
def x
  20
end
puts square.call()
# => 400

binding is smart enough to figure out that since no x variable is present，这里方法可以后定义。

• lexical binding in procs

def square(p)
   x = 2
   puts p.call
end
x = 20
square(lambda { x * x })
#=> 400

In the above case the value of x is set as 20 at the code compile time. Don’t get fooled by x being 2 inside the method call. Inside the method call a new scope starts and the x inside the method is not the same x as outside .

proc只跟当前上下文的变量绑定

• Issues because of lexical scoping

class Person
  code = proc { puts self }

  define_method :name do
    code.call()
  end
end

class Developer < Person
end

Person.new.name # => Person
Developer.new.name # => Person

In the above case when Developer.new.name is executed then output is Person. And that can cause problem. For example in Ruby on Rails at a number of places self is used to determine if the model that is being acted upon is STI or not. If the model is STI then for Developer the query will have an extra where clause like AND "people"."type" IN ('Developer') . So we need to find a solution so that self reports correctly for both Person and ‘Developer` .

• instance_eval can change self

class Person
  code = proc { puts self }

  define_method :name do
    self.class.instance_eval &code
  end
end

class Developer < Person
end

Person.new.name #=> Person
Developer.new.name #=> Developer

但是instance_eval不能接受参数

class Person
  code = proc { |greetings| puts greetings; puts self }

  define_method :name do
    self.class.instance_eval 'Good morning', &code
  end
end

class Developer < Person
end

Person.new.name
Developer.new.name

#=> wrong number of arguments (1 for 0) (ArgumentError)

• instance_exec to rescue

class Person
  code = proc { |greetings| puts greetings; puts self }

  define_method :name do
    self.class.instance_exec 'Good morning', &code
  end
end

class Developer < Person
end

Person.new.name #=> Good morning Person
Developer.new.name #=> Good morning Developer