Added enum contract

TUTORIAL.md

@@ -88,6 +88,7 @@ contracts.ruby comes with a lot of built-in contracts, including the following:
   * [`SetOf`](http://www.rubydoc.info/gems/contracts/Contracts/SetOf) – checks that the argument is a set, and all elements pass the given contract, e.g. `SetOf[Num]`
   * [`HashOf`](http://www.rubydoc.info/gems/contracts/Contracts/HashOf) – checks that the argument is a hash, and all keys and values pass the given contract, e.g. `HashOf[Symbol => String]` or `HashOf[Symbol,String]`
   * [`RangeOf`](http://www.rubydoc.info/gems/contracts/Contracts/RangeOf) – checks that the argument is a range whose elements (#first and #last) pass the given contract, e.g. `RangeOf[Date]`
+  * [`Enum`](http://www.rubydoc.info/gems/contracts/Contracts/Enum) – checks that the argument is part of a given collection of objects, e.g. `Enum[:a, :b, :c]`
 
 * Keyword arguments
   * [`KeywordArgs`](http://www.rubydoc.info/gems/contracts/Contracts/KeywordArgs) – checks that the argument is an options hash, and all required keyword arguments are present, and all values pass their respective contracts, e.g. `KeywordArgs[:number => Num, :description => Optional[String]]`

lib/contracts/builtin_contracts.rb

@@ -201,6 +201,20 @@ def to_s
     end
   end
 
+  # Takes a list of values, e.g. +[:a, :b, :c]+. If argument is included in
+  # the list, the contract passes.
+  #
+  # Example: <tt>Enum[:a, :b, :c]</tt>?
+  class Enum < CallableClass
+    def initialize(*vals)
+      @vals = vals
+    end
+
+    def valid?(val)
+      @vals.include? val
+    end
+  end
+
   # Takes a value +v+. If the argument is +.equal+ to +v+, the contract passes,
   # otherwise the contract fails.
   # Example: <tt>Eq[Class]</tt>

spec/builtin_contracts_spec.rb

@@ -164,6 +164,16 @@
     end
   end
 
+  describe "Enum:" do
+    it "should pass for an object that is included" do
+      expect { @o.enum_test(:a) }.to_not raise_error
+    end
+
+    it "should fail for an object that is not included" do
+      expect { @o.enum_test(:z) }.to raise_error(ContractError)
+    end
+  end
+
   describe "RespondTo:" do
     it "should pass for an object that responds to :good" do
       expect { @o.responds_test(A.new) }.to_not raise_error

spec/fixtures/fixtures.rb

@@ -200,6 +200,10 @@ def xor_test(x)
   def and_test(x)
   end
 
+  Contract Enum[:a, :b, :c] => nil
+  def enum_test(x)
+  end
+
   Contract RespondTo[:good] => nil
   def responds_test(x)
   end