pithyless · March 27, 2012 14:44 · Mar 27, 2012 · Mar 27, 2012 · Mar 27, 2012 · Mar 27, 2012
diff --git a/examples.rb b/examples.rb
@@ -0,0 +1,16 @@
+
+def controller
+
+  res = ParseParam.call(params)
+  res = res >> lambda { |res| DoSomeStuff.call(res) }
+  res = res >> lambda { |res| FinickyThirdParty.call(res) }
+  res = res >> ...
+  res.to_xml
+
+end
+
+# If this seems like a good idea, we can add a little bit of sugar:
+
+res = Either.right('ok')
+res >>= JustAnotherProc
+
diff --git a/either.rb b/either.rb
@@ -36,6 +36,26 @@ def >>(callable)
     end
   end
 
+  # Short-circuit applicative AND
+  #
+  # Examples
+  #
+  #   Either.right(1) & Either.right(2) & Either.right(3)
+  #   #=> #<Either: @left=nil, @right=3>
+  #
+  #   Either.right(1) & Either.left(2) & Either.right(3)
+  #   #=> #<Either: @left=2, @right=nil>
+  #
+  # Returns either the first Left or the last Right
+  def &(other)
+    fail "Expected Either; got #{other.inspect}" unless other.is_a?(Either)
+    if left?
+      self
+    else
+      other
+    end
+  end
+
   def self.left(left)
     new(left, nil)
   end

diff --git a/either.rb b/either.rb
@@ -28,7 +28,7 @@ def right?
 
   def >>(callable)
     if left?
-      Either.left(left)
+      self
     else
       callable.call(right).tap do |e|
         fail "No quantum leaps allowed! Expected Either; got #{e.inspect}" unless e.is_a?(Either)

diff --git a/either.rb b/either.rb
@@ -0,0 +1,47 @@
+# A chainable Either monad for Ruby
+#
+# Examples
+#
+#  Either.right('s') >> proc { |s| Either.right(s + '-1') } >> proc { |s| Either.right(s + '-2') }
+#  #=> #<Either @left=nil, @right="s-1-2">
+#
+#  Either.right('s') >> proc { |s| Either.left('error!') } >> proc { |s| Either.right(s + '-2') }
+#  #=> #<Either @left='error!', @right=nil>
+#
+# Returns either left or right.
+class Either
+  attr_reader :left, :right
+  private_class_method :new
+
+  def initialize(left, right)
+    @left = left
+    @right = right
+  end
+
+  def left?
+    !!left
+  end
+
+  def right?
+    not left?
+  end
+
+  def >>(callable)
+    if left?
+      Either.left(left)
+    else
+      callable.call(right).tap do |e|
+        fail "No quantum leaps allowed! Expected Either; got #{e.inspect}" unless e.is_a?(Either)
+      end
+    end
+  end
+
+  def self.left(left)
+    new(left, nil)
+  end
+
+  def self.right(right)
+    new(nil, right)
+  end
+
+end
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