First

.gitignore

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+*.gem
+*.rbc
+.bundle
+.config
+.yardoc
+Gemfile.lock
+InstalledFiles
+_yardoc
+coverage
+doc/
+lib/bundler/man
+pkg
+rdoc
+spec/reports
+test/tmp
+test/version_tmp
+tmp

Gemfile

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+source 'https://rubygems.org'
+
+# Specify your gem's dependencies in ruby_ukanren.gemspec
+gemspec

LICENSE.txt

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+Copyright (c) 2014 Justin Leitgeb
+
+MIT License
+
+Permission is hereby granted, free of charge, to any person obtaining
+a copy of this software and associated documentation files (the
+"Software"), to deal in the Software without restriction, including
+without limitation the rights to use, copy, modify, merge, publish,
+distribute, sublicense, and/or sell copies of the Software, and to
+permit persons to whom the Software is furnished to do so, subject to
+the following conditions:
+
+The above copyright notice and this permission notice shall be
+included in all copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
+EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
+NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE
+LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
+OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
+WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

README.md

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+# RubyUkanren
+
+TODO: Write a gem description
+
+## Installation
+
+Add this line to your application's Gemfile:
+
+    gem 'ruby_ukanren'
+
+And then execute:
+
+    $ bundle
+
+Or install it yourself as:
+
+    $ gem install ruby_ukanren
+
+## Usage
+
+TODO: Write usage instructions here
+
+## Contributing
+
+1. Fork it ( http://github.com/<my-github-username>/ruby_ukanren/fork )
+2. Create your feature branch (`git checkout -b my-new-feature`)
+3. Commit your changes (`git commit -am 'Add some feature'`)
+4. Push to the branch (`git push origin my-new-feature`)
+5. Create new Pull Request

Rakefile

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+require "bundler/gem_tasks"
+
+require 'rake/testtask'
+
+Rake::TestTask.new do |t|
+  t.libs << "spec"
+  t.test_files = FileList['spec/**/*_spec.rb']
+end
+
+task :default => :test

lib/ukanren.rb

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+require "ukanren/version"
+require "ukanren/lisp"
+require "ukanren/language"

lib/ukanren/language.rb

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+module Ukanren
+  module Language
+    include Lisp
+
+    def var(*c)       ; Array.new(c)   ; end
+    def var?(x)       ; x.is_a?(Array) ; end
+
+    def vars_eq?(x1, x2) ; x1 == x2    ; end
+
+    def mzero ; nil ; end
+
+    def ext_s(x, v, s)
+      cons(cons(x, v), s)
+    end
+
+    def unify(u, v, s)
+      u = walk(u, s)
+      v = walk(v, s)
+
+      if var?(u) && var?(v) && vars_eq?(u, v)
+        s
+      elsif var?(u)
+        ext_s(u, v, s)
+
+      elsif var?(v)
+        ext_s(v, u, s)
+
+      elsif pair?(u) && pair?(v)
+        if s = unify(car(u), car(v), s)
+          unify(cdr(u), cdr(v), s)
+        end
+      elsif u == v
+        s
+      end
+    end
+
+    # -- Constrain u to be equal to v.
+    def eq(u, v)
+      ->(s_c) {
+        s = unify(u, v, car(s_c))
+        s ? unit(cons(s, cdr(s_c))) : mzero
+      }
+    end
+
+    # Walk environment S and look up value of U, if present.
+    def walk(u, s)
+      if var?(u) then
+        pr = assp(-> (v) { u == v }, s)
+        pr ? walk(cdr(pr), s) : u
+
+      else
+        u
+      end
+    end
+
+    # Call function f with a fresh variable.
+    def call_fresh(f)
+      -> (s_c) {
+        c = cdr(s_c)
+        f.call(var(c)).call(cons(car(s_c), c + 1))
+      }
+    end
+
+    def mplus(d1, d2)
+      if d1.nil?
+        d2
+      elsif d1 == mzero
+        d2
+      elsif d1.is_a?(Proc)
+        -> { mplus(d2, d1.call) }
+      else
+        cons(car(d1), mplus(cdr(d1), d2))
+      end
+    end
+
+    def bind(d, g)
+      if d.nil?
+        mzero
+      elsif d == mzero
+        mzero
+      elsif d.is_a?(Proc)
+        -> { bind(d.call, g) }
+      else
+        mplus(g(car(d)), bind(cdr(d), g))
+      end
+    end
+
+    def disj(g1, g2)
+      -> (s_c) {
+        mplus(g1(s_c), g2(s_c))
+      }
+    end
+
+    def conj(g1, g2)
+      -> (s_c) {
+        bind(g1(s_c), g2)
+      }
+    end
+
+    def empty_env
+      cons(mzero, 0)
+    end
+  end
+end

lib/ukanren/lisp.rb

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+module Ukanren
+  module Lisp
+
+    def cons(x, y) ; -> (m) { m.call(x, y) } ; end
+    def car(z)     ; z.call(-> (p, q) { p }) ; end
+    def cdr(z)     ; z.call(-> (p, q) { q }) ; end
+
+    # Search association list by predicate function.
+    # Based on lua implementation by silentbicycle:
+    # https://github.com/silentbicycle/lua-ukanren/blob/master/ukanren.lua#L53:L61
+    #
+    # Additional reference for this function is scheme:
+    # Ref for assp: http://www.r6rs.org/final/html/r6rs-lib/r6rs-lib-Z-H-4.html
+    def assp(func, alist)
+      if alist && hd = car(alist)
+        func.call(hd) ? hd : assp(func, cdr(alist))
+      end
+    end
+
+  end
+end

lib/ukanren/version.rb

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+module Ukanren
+  VERSION = "0.0.1"
+end

ruby_ukanren.gemspec

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+# coding: utf-8
+lib = File.expand_path('../lib', __FILE__)
+$LOAD_PATH.unshift(lib) unless $LOAD_PATH.include?(lib)
+require 'ukanren/version'
+
+Gem::Specification.new do |spec|
+  spec.name          = "ruby_ukanren"
+  spec.version       = Ukanren::VERSION
+  spec.authors       = ["Justin Leitgeb"]
+  spec.email         = ["[email protected]"]
+  spec.summary       = %q{uKanren in Ruby}
+  spec.description   = %q{Implements uKanren paper in Ruby}
+  spec.homepage      = ""
+  spec.license       = "MIT"
+
+  spec.files         = `git ls-files -z`.split("\x0")
+  spec.executables   = spec.files.grep(%r{^bin/}) { |f| File.basename(f) }
+  spec.test_files    = spec.files.grep(%r{^(test|spec|features)/})
+  spec.require_paths = ["lib"]
+
+  spec.add_development_dependency "bundler", "~> 1.5"
+  spec.add_development_dependency "rake"
+  spec.add_development_dependency 'mocha'
+end

spec/spec_helper.rb

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+require 'minitest/autorun'
+
+require "mocha"
+
+require "#{File.dirname(__FILE__)}/../lib/ukanren"

spec/ukanren/language_spec.rb

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+require 'spec_helper'
+
+describe Ukanren::Language do
+  include Ukanren::Language
+
+  describe "second-set t1" do
+    it "returns one binding" do
+      call_fresh(-> (q) { eq(q, 5) }).call(empty_env)
+    end
+  end
+end

spec/ukanren/lisp_spec.rb

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+require 'spec_helper'
+
+describe Ukanren::Lisp do
+  include Ukanren::Lisp
+
+  describe "#car" do
+    it "returns the first element in the pair" do
+      car(cons(1, 2)).must_equal 1
+    end
+  end
+
+  describe "#cdr" do
+    it "returns the second item in the pair" do
+      cdr(cons(1, 2)).must_equal 2
+    end
+  end
+
+  describe "#cons" do
+    it "returns a pair" do
+      car(cons(1, 2)).must_equal 1
+      cdr(cons(1, 2)).must_equal 2
+    end
+  end
+
+  describe "#assp" do
+    it "returns the first element for which the predicate function is true" do
+      alist = cons(1, cons(2, cons(3, 4)))
+      assp(->(i) { i == 3 }, alist).must_equal 3
+    end
+  end
+end