Elixir build custom function with pattern matching and guards using macros?

Question

I need to take a DSL like this:

defmodule SomeModule do
  use SomeMacros

  # sample shipping DSL
  rule is_north_america do
    calculate_shipping_cost_with usps
  end

  rule is_north_america and november_or_december do
    calculate_shipping_cost_with ups
  end

  rule is_south_america do
    calculate_shipping_cost_with fedex
  end

  rule is_somewhere_else do
    calculate_shipping_cost_with dhl
  end
end

And convert each call to the rule macro(which I already have defined) and have it define a function within SomeModule module. Like this:

def is_north_america_rule(Address[continent: "North America"] = address) do
  # do something
end

I would like to use pattern matching on the parameters passed into the functions generated by the macros. I have seen how to define custom functions in macros but I am unsure as how to go about implementing pattern matching and guards in functions generated by macros.

Thanks in advance!

José Valim José Valim · Accepted Answer · 2013-11-12T14:53:12

Here is an example of how you could achieve something similar:

https://gist.github.com/josevalim/7432084

I was going to add more information but I decided to provide a simple example and then answer any further question you may have. As a note though, try to make this rules DSL as simple as possible, if you see the macro code starting to get more and more complex, simply using a cond (explicitly) would be better.

Elixir build custom function with pattern matching and guards using macros?

1 Answers