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POConstants in classes created with Class.new
 

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  1. POConstants in classes created with Class.new
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    <p>I'm writing a little piece of code in Ruby (1.9.3), and I use a pair of simple "enum-like" classes, that define some constants with <strong>const_set</strong> and some behavior of these constants (e.g. the class <strong>Days</strong> may have the constants <strong>MON</strong>, <strong>TUE</strong>... and <strong>Days::MON.succ</strong> should evaluate to <strong>TUE</strong>).</p> <p>I'm really comfortable with these classes. However, while growing my code, i sometimes need to add more of them, and I don't like the idea of having five or more classes that share 99% of source code, e.g.:</p> <pre><code>class Days NAMES = %w( MON TUE ... ) INSTANCES = [] def initialize(num) @num = num end # An example operation def +(n) INSTANCES[(@num + n) % INSTANCES.length] end # Another example operation def succ self + 1 end def to_s NAMES[@num] end NAMES.each_with_index do |name, idx| instance = new(idx) INSTANCES[idx] = instance const_set name, instance end end class Months NAMES = %w( JAN FEB ... ) ... end </code></pre> <p>I was wondering if Ruby's metaprogramming capabilities could be used to generate these classes. However, I'm having an hard time creating <strong>NAMES</strong>, <strong>INSTANCES</strong> and the "enum-named" constants (e.g. <strong>MON</strong>, <strong>TUE</strong>, ...). Being <strong>const_set</strong> a class method of <strong>Class</strong>, in this code it's context (the value of <strong>self</strong>) is respectively <strong>Days</strong> and <strong>Months</strong>.</p> <p>When creating a factory method, I'm compelled to do something like this:</p> <pre><code>def enum_new(names_array) Class.new do const_set "NAMES" [] names_array.each_with_index do |name, idx| NAMES[idx] = name end ... end end Days = enum_new(%w| MON TUE ... |) Months = enum_new(%w| JAN FEB ... |) </code></pre> <p>but this won't work (at least, not like i hoped it to), because <strong>const_set</strong> won't be called in the context of the class whose name is <em>magically</em> set (i.e. <strong>Days</strong> and <strong>Months</strong>), but apparently in the context of <strong>Class</strong>; therefore, not only it won't be accessible from the instance methods, but it will be overwrited every time <strong>enum_new</strong> is called with a new array of names as argument. A similar problem shows up when using class variables, because they'll be shared between any class generated with the method (because they'll become class variables of <strong>Class</strong>, i guess).</p> <p>Is there any way to create constants in a class generated with <strong>Class.new</strong>, obtaining this way classes identical in everything to the original <strong>Days</strong> and <strong>Months</strong> classes, without having to pollute my code with almost identical classes?</p> <p>Thanks for your attention and patience! :)</p>
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    1. USAlberto Moriconi
    1. USAlberto Moriconi
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    1. COCan you please fill in the code you emit with your elipsis?
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      1. USLinuxios
    2. COI completed the code of the first class and also added two example operations to show how I need to access the constants in instance methods. Of course instances are accessed via the constants created with const_set.
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      1. POConstants in classes created with Class.new
      1. USAlberto Moriconi
 

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