fib.rb

require 'pattern'

# Returns the n'th number in the Fibonacci sequence.
def fib(n)
  Pattern.match(n) do
    on(0) { 0 }
    on(1) { 1 }
    on(Integer) { fib(n - 2) + fib(n - 1) }
  end
end

pattern.rb

# A simple implementation of pattern matching in Ruby.
class Pattern
  class NoMatch < RuntimeError
  end

  def self.match(value, &block)
    catch(:ok) do
      matcher = Matcher.new(value)
      matcher.instance_eval(&block)

      # If none of the patterns matched, we'll end up here.
      raise NoMatch
    end
  end
end

class Pattern::Matcher
  def initialize(value)
    @value = value
  end

  def on(pattern)
    if pattern === @value
      throw :ok, yield
    end
  end
end

pattern_test.rb

require 'rubygems'
require 'rspec/autorun'

describe Pattern::Matcher do
  describe "#on" do
    let(:matcher) { Pattern::Matcher.new("foo") }

    it "throws :ok when the pattern matches the value" do
      catch(:ok) { matcher.on(/foo/) { "bar" } }.should eq "bar"
    end

    it "does not throw :ok when the pattern doesn't match the value" do
      matcher.on("baz") { "bar" }.should be_nil
    end
  end
end

describe Pattern, ".match" do
  it "returns the value from the matching clause" do
    Pattern.match "foo" do
      on("foo") { "bar" }
      on("baz") { "bim" }
    end.should eq "bar"
  end

  it "returns nil if that's what the matching clause returns" do
    Pattern.match "foo" do
      on("foo") { nil }
    end.should be_nil
  end

  it "raises NoMatch if there were no matches" do
    lambda do
      Pattern.match "foo" do
        on("baz") { "bar" }
      end
    end.should raise_error(Pattern::NoMatch)
  end
end