Initial commit

.vscode/settings.json

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+{
+  "code-runner.customCommand": "ruby test/mutual_recursion_test.rb",
+  "code-runner.runInTerminal": true
+}

lib/mutual_recursion.rb

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+# frozen_string_literal: true
+
+module MutualRecursion
+  UNEXPECTED_TYPE = 'expected tail_call or terminal_value'
+
+  class TailCall
+    attr_reader :value, :block
+
+    def initialize(value = nil, &block)
+      @value = value
+      @block = block
+    end
+
+    def invoke
+      self.then do |tail|
+        while tail.block
+          tail = tail.block.call
+          raise TypeError, UNEXPECTED_TYPE unless tail.is_a?(TailCall)
+        end
+
+        tail.value
+      end
+    end
+  end
+end
+
+def tail_call
+  MutualRecursion::TailCall.new { yield }
+end
+
+def terminal_value(value)
+  MutualRecursion::TailCall.new(value)
+end

mutual_recursion.gemspec

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+# frozen_string_literal: true
+
+Gem::Specification.new do |s|
+  s.name        = 'mutual_recursion'
+  s.version     = '0.0.1'
+  s.date        = '2019-01-26'
+  s.summary     = 'Mutual Recursion for Ruby'
+  s.description = 'Tail call optimization for mutually (and directly) recursive functions using a trampoline.'
+  s.authors     = ['Mike']
+  s.files       = ['lib/mutual_recursion.rb']
+  s.homepage    = ''
+  s.license     = 'MIT'
+end

test/mutual_recursion_test.rb

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+# frozen_string_literal: true
+
+require 'minitest/autorun'
+require 'minitest/pride'
+
+require_relative '../lib/mutual_recursion'
+
+class InventoryTest < Minitest::Test
+  def test_terminal_value
+    tail = terminal_value(42)
+    assert_equal(42, tail.invoke)
+  end
+
+  def test_single_recursion
+    tail = tail_call { terminal_value(42) }
+    assert_equal(42, tail.invoke)
+  end
+
+  def test_several_recursions
+    tail = tail_call { tail_call { tail_call { terminal_value(42) } } }
+    assert_equal(42, tail.invoke)
+  end
+
+  def test_mutual_tail_recursion
+    tail = one(50_000)
+    assert_equal(50_001, tail.invoke)
+  end
+
+  def test_direct_tail_recursion
+    tail = direct(50_000)
+    assert_equal(50_001, tail.invoke)
+  end
+
+  def test_recursive_function_can_return_lambda
+    tail = lambda_returning
+    assert_kind_of(Proc, tail.invoke)
+  end
+
+  def test_recursive_function_can_return_proc
+    tail = proc_returning
+    assert_kind_of(Proc, tail.invoke)
+  end
+
+  def test_non_tail_call_detected
+    tail = bad_return
+    assert_raises(TypeError) { tail.invoke }
+  end
+end
+
+class TailCallDecoy
+  attr_reader :value, :block
+
+  def initialize
+    @value = 99
+    @block = proc { 25 }
+  end
+end
+
+def one(x, y = 0)
+  return terminal_value(y) if x.negative?
+
+  tail_call { two(x, y + 1) }
+end
+
+def two(x, y)
+  tail_call { one(x - 1, y) }
+end
+
+def direct(x, y = 0)
+  return terminal_value(y) if x.negative?
+
+  tail_call { direct(x - 1, y + 1) }
+end
+
+def lambda_returning
+  terminal_value(-> { 24 })
+end
+
+def proc_returning
+  terminal_value(proc { 24 })
+end
+
+def bad_return
+  tail_call { TailCallDecoy.new }
+end