honor Func contract on returns (fixes issue #143)

lib/contracts.rb

@@ -383,6 +383,10 @@ def call_with(this, *args, &blk)
 
     this.verify_invariants!(method) if this.respond_to?(:verify_invariants!)
 
+    if ret_contract.is_a?(Contracts::Func)
+      result = Contract.new(klass, result, *ret_contract.contracts)
+    end
+
     result
   end
 

spec/contracts_spec.rb

@@ -509,6 +509,18 @@ def self.greeting(name)
     it "should fail for a function that doesn't pass the contract with weak other args" do
       expect { @o.map_plain(["hello", "joe"], lambda { |_| nil }) }.to raise_error(ContractError)
     end
+
+    it "should fail for a returned function that doesn't pass the contract" do
+      expect { @o.lambda_with_wrong_return.call("hello") }.to raise_error(ContractError)
+    end
+
+    it "should fail for a returned function that receives the wrong argument type" do
+      expect { @o.lambda_with_correct_return.call(123) }.to raise_error(ContractError)
+    end
+
+    it "should not fail for a returned function that passes the contract" do
+      expect { @o.lambda_with_correct_return.call("hello") }.to_not raise_error
+    end
   end
 
   describe "default args to functions" do

spec/fixtures/fixtures.rb

@@ -267,6 +267,16 @@ def map_plain(arr, func)
     end
   end
 
+  Contract None => Func[String => Num]
+  def lambda_with_wrong_return
+    lambda { |x| x }
+  end
+
+  Contract None => Func[String => Num]
+  def lambda_with_correct_return
+    lambda { |x| x.length }
+  end
+
   Contract Num => Num
   def default_args(x = 1)
     2