Pattern matching for Ruby methods

Add this line to your application's Gemfile:

gem 'method_pattern'

And then execute:

$ bundle

Or install it yourself as:

$ gem install method_pattern

Extend any class you want to be able to use pattern matching with the MethodPattern module:

class HandleResponse
  extend MethodPattern
end

Then you can define your method with defn:

class HandleResponse
  extend MethodPattern

  defn :call do
    # ...
  end
end

Inside of your defn block, you declare your argument patterns using with:

class Fibonnaci
  extend MethodPattern

  defn :call do
    with(0..1) { |n| n }
    with(Integer) { |n| call(n - 1) + call(n - 2) }
  end
end

This example will handle 0 and 1 as special cases and all other integers are funneled into the second implementation. Patterns declared higher take precedence.

Notice that we could pass in a class or even a range for our pattern. There are several things we can use:

• Strings: with('hello') { |str| ... } matches an exact string
• Numbers: with(15) { |num| ... } matches an exact number
• Symbol: with(:foo) { |sym| ... } matches a particular symbol
• Class: with(Integer) { |num| ... } matches any instance of the given class
• Regex: with(/foo/) { |str| ... } matches any string that matches the regex
• Range: with(0...10) { |num| ... } matches any value covered by the range
• Proc/lambda: with(-> n { n > 3 }) { |n| ... } matches if the proc returns a truthy value

Note that the method arguments are passed to the block. This lets the block become the method body.

You can pass multiple patterns to with and it will match them in order:

defn :baz do
  with('foo', /bar/) { |a, b| a + b }
end

Keyword arguments work, too:

class HandleResponse
  extend MethodPattern

  defn :call do
    with status: 200...300, headers: { 'Content-Type': /json/ } do |body:, **|
      JSON.parse(body, symbolize_names: true)
    end

    # All 4xx and 5xx responses are errors
    with(status: 400..599) { |body:, **| ErrorResponse.call body }
  end
end

Unfortunately, because with accepts its own block, you cannot match on whether a block was passed to the method.

After checking out the repo, run bin/setup to install dependencies. Then, run rake spec to run the tests. You can also run bin/console for an interactive prompt that will allow you to experiment.

To install this gem onto your local machine, run bundle exec rake install. To release a new version, update the version number in version.rb, and then run bundle exec rake release, which will create a git tag for the version, push git commits and tags, and push the .gem file to rubygems.org.

Bug reports and pull requests are welcome on GitHub at https://github.com/jgaskins/method_pattern. This project is intended to be a safe, welcoming space for collaboration, and contributors are expected to adhere to the Contributor Covenant code of conduct.

The gem is available as open source under the terms of the MIT License.

Everyone interacting in the MethodPattern project’s codebases, issue trackers, chat rooms and mailing lists is expected to follow the code of conduct.