Execution block error

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?

Isn't it doing precisely what you want? You're validating that age is >= 18, and then you use an age of 10 and an exception is raised. What's surprising? — Frederick Cheung
– Frederick Cheung, Commented Mar 20, 2012 at 8:27

Nikhil · Accepted Answer · 2012-03-20 09:32:50Z

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)

edited Mar 20, 2012 at 9:32

answered Mar 20, 2012 at 8:16

Nikhil

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