Identity monad

lib/do_notation/monads/identity.rb

@@ -0,0 +1,14 @@
+module Identity
+  include Monad
+
+  class << self
+    def unit(obj)
+      obj
+    end
+
+    def bind(obj, &fn)
+      fn.call(obj)
+    end
+  end
+end
+

spec/identity_spec.rb

@@ -0,0 +1,27 @@
+require File.join(File.dirname(__FILE__), 'spec_helper')
+require 'do_notation/monads/identity'
+
+describe "Identity:" do
+  specify "just works like sequential code" do
+    value = Identity.run do
+      x <- 1
+      y <- 2
+
+      x + y
+    end
+
+    value.should == 3
+  end
+
+  specify "doesn't do anything special with nil" do
+    lambda {
+      value = Identity.run do
+        x <- 1
+        y <- nil
+
+        x + y
+      end
+    }.should raise_error(TypeError)
+  end
+end
+

spec/monad_spec.rb

@@ -28,8 +28,9 @@
       unit(x+y)
     end
 
-    Array.run(&block).should == Array.unit(3)
-    Maybe.run(&block).should == Maybe.unit(3)
+    [Identity, Maybe, Array].each do |monad|
+      monad.run(&block).should == monad.unit(3)
+    end
   end
 
   specify "should be nestable" do