A new REPL on Replit to help demonstrate three techniques to safely share values between modules. DRY up your code in a safe way!

defmodule MyMath do
@moduledoc """
This repl documents three ways to share values between modules.  

The question may occur to the reader--why would I want to share values between modules?  It's a valid question.  It is often the case that we need to share some common piece of logic among different contexts.  For example if we use a regular expression (regex) to parse, for example, an address, we want to insure we use the same regex every place we need to parse an address.  We can certainly copy/paste the regex but that leads to a lot more work for later developers who if they find they need to modify the regex must insure they find all the uses of the regex and modify all of them.  This becomes an even larger issue when you have teams of developers, potentially distributed across geography.  Communication is already a problem in that case and it's far more likely that someone will miss something. 

The following repl is structured as one file with two modules to make it easy to run and observe the behavior.  The Main module gives examples of how to use the shared values across modules. 

You can try the following in the console to assure yourself that this all works as expected:

## Examples

iex>  radius = 3.0
3.0
iex>  Main.circle_area(radius) == Main.circle_area_alternate(radius)
true
iex>  Main.circle_area_macro_approach(radius) == Main.circle_area(radius)
true
"""

   #First symbolic constant technique: module attribute
   Module.register_attribute(__MODULE__, :pi, persist: true)
   @pi 3.141592653

   #Second symbolic constant technique: function to return constant value
   def pi, do: 3.141592653
   # this could also be: 
   # def pi, do: @pi

   #Third symbolic constant technique (many thanks to Paul Schoenfelder for this): macro 
   defmacro __using__(_) do
    quote do
       @pi 3.141592653
    end
  end
end

defmodule Main do
   defp get_pi do # module attribute approach
      [pi] = MyMath.__info__(:attributes)[:pi]
      pi
   end

   def circle_area(radius) when is_float(radius) do
      MyMath.pi() * radius * radius #constant function approach
   end

   # or
   def circle_area_alternate(radius) when is_float(radius) do
      get_pi() * radius * radius
   end

   # or
   use MyMath

   def circle_area_macro_approach(radius) when is_float(radius) do
    @pi * radius * radius
   end

end

Try it out on Replit here!