Question

I have the next code:

class Class
    def attr_checked(attribute, &validation)
        define_method "#{attribute}=" do |value|            
            raise 'Invalid attribute' unless validation.call(value)         
            instance_variable_set("@#{attribute}", value)
        end

        define_method attribute do
            instance_variable_get "@#{attribute}"
        end
    end
end

class Person
    attr_checked :age do |v|
        v >= 18
    end
end

bob = Person.new
bob.age = 10
p bob.age

and the error message when I am executing it:

.\example_19.rb ./example_19.rb:4:in block in attr_checked':
Invalid attribute (RuntimeError)
from ./example_19.rb:23:in‘

Why and how can I fix it?

Answer 1

This is actually doing exactly what your code asks.

The attr_checked method only returns true if the block evaluates to true. Your block, returns true only if the age is greater than or equal to 18.

attr_checked :age do |v|
        v >= 18
end

When you set age = 10, this block returns false and the ‘Invalid Attribute’ error is returned according to this line:

raise 'Invalid attribute' unless validation.call(value)