Created
April 9, 2013 17:04
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Couldn't sleep, so I added some monads to Ruby
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| require "minitest/autorun" | |
| module Monad | |
| def return(val) | |
| self.class.new(val) | |
| end | |
| def bind(&block) | |
| self.return yield @value | |
| end | |
| end | |
| class Identity | |
| include Monad | |
| attr_reader :value | |
| def initialize(value) | |
| @value = value | |
| self | |
| end | |
| def ==(value) | |
| @value == value | |
| end | |
| end | |
| describe Identity do | |
| it "#bind is a left-identity with respect to return" do | |
| value = rand(100000) | |
| monad = Identity.new(value) | |
| monad.bind do |v| | |
| Identity.new(v) | |
| end.must_equal(monad) | |
| end | |
| it "#return is a right-identity with respect to bind" do | |
| value = rand(100000) | |
| rand1 = rand(100000) | |
| f = lambda { |v| v + rand1 } | |
| monad = Identity.new(value) | |
| monad.bind(&f).must_equal f.call(value) | |
| end | |
| it "#bind is associative" do | |
| value = rand(100000) | |
| rand1 = rand(100000) | |
| rand2 = rand(100000) | |
| f = lambda { |v| v + rand1 } | |
| g = lambda { |v| v + rand2 } | |
| monad = Identity.new(value) | |
| monad.bind do | |
| f.call(g.call(value)) | |
| end.must_equal monad.bind(&f).bind(&g) | |
| end | |
| end | |
| module Just | |
| def just | |
| self | |
| end | |
| end | |
| class Nothing | |
| def self.just | |
| self | |
| end | |
| def just | |
| self.class | |
| end | |
| end | |
| class Object | |
| include Just | |
| end | |
| class Maybe | |
| include Monad | |
| attr_reader :value | |
| def initialize(value) | |
| @value = value | |
| end | |
| def fetch | |
| @value.just | |
| end | |
| def bind(&block) | |
| self.return case fetch | |
| when Nothing then Nothing | |
| else yield @value | |
| end | |
| end | |
| def ==(value) | |
| fetch == value | |
| end | |
| end | |
| describe Maybe do | |
| it "#bind is a left-identity with respect to return" do | |
| value = rand(100000) | |
| monad = Maybe.new(value) | |
| monad.bind do |v| | |
| Maybe.new(v) | |
| end.must_equal(monad) | |
| end | |
| it "#return is a right-identity with respect to bind" do | |
| value = rand(100000) | |
| rand1 = rand(100000) | |
| f = lambda { |v| v + rand1 } | |
| monad = Maybe.new(value) | |
| monad.bind(&f).must_equal f.call(value) | |
| end | |
| it "#bind is associative" do | |
| value = rand(100000) | |
| rand1 = rand(100000) | |
| rand2 = rand(100000) | |
| f = lambda { |v| v + rand1 } | |
| g = lambda { |v| v + rand2 } | |
| monad = Maybe.new(value) | |
| monad.bind do | |
| f.call(g.call(value)) | |
| end.must_equal monad.bind(&f).bind(&g) | |
| end | |
| end |
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