Metaprogramming errors in Ruby - ruby

Question

Alright, so this is driving me crazy. The point of this code is that I ought to be able to add a method dynamically provided it is of the form object.plusnum, where num is any number. I'm not quite sure how to get this to work. This is my best shot at it so far, but I'm currently getting several errors.
Code:
class Adder
def initialize(_val)
#start_value = _val
end
def method_missing(method_name, *args)
method = method_name.to_s
if method.start_with?("plus") then
num = method[4 .. method.length]
if (/^[\d]+(\.[\d]+){0,1}$/ === num) then
number = Integer(num)
self.class_eval("def #{method} return #start_value + #{number} end")
else
super
end
else
super
end
end
end
The error I'm currently getting is that "class_eval" is undefined. I am pretty new to metaprogramming and ruby, and this is driving me crazy.

Answer 1

I think you got it all wrong :)
Calling a method for the first time yields different result, than calling it second time so probably you would like this method to be called instantly after defining. Also -- you are using fairly complex regex and then converting value to Integer and dropping all digits after dot.
You are using class_eval and passing string to it, which is usually bad idea, and block should be used whenever possible, for security and performance reasons.
How I see it could look:
class Adder
def initialize(val)
#start_value = val
end
def method_missing(method_name, *args)
if method_name.to_s =~ /^plus(\d+)$/
self.class.class_eval do
define_method(method_name) { #start_value + $1.to_i }
end
self.send(method_name)
else
super
end
end
end

Answer 2

class Adder
def initialize(_val)
#start_value = _val
end
def method_missing(method_name, *args)
method = method_name.to_s
if method.start_with?("plus") then
num = method[4 .. method.length]
if (/^[\d]+(\.[\d]+){0,1}$/ === num) then
number = Integer(num)
self.class.class_eval("def #{method}() return #start_value + #{number} end")
eval(method)
else
super
end
else
super
end
end
end
a = Adder.new(0)
a.plus1
Make sure to add eval(method) in the end to call the method otherwise it will return nil for just creating the method. Or you can simple return with return #start_value + #{number}

Answer 3

You have to call class_eval on the Adder class, not on an instance of adder.
The string isn't valid Ruby. Put parentheses after #{method}.
The new version of the code:
class Adder
def initialize(_val)
#start_value = _val
end
def method_missing(method_name, *args)
method = method_name.to_s
if method.start_with?("plus") then
num = method[4 .. method.length]
if (/^[\d]+(\.[\d]+){0,1}$/ === num) then
number = Integer(num)
self.class.class_eval("def #{method}() return #start_value + #{number} end")
else
super
end
else
super
end
end
end
a = Adder.new(0)
a.plus1
Speaking for myself, the way I would have built this method up is just to start off with
class Adder
end
Adder.class_eval("def plus1() return 0 + 1 end")
a = Adder.new
a.plus1
and then gradually replaced the hard-wired values with configurable values, rather than writing everything at once.