Fn::ImportValue support

Add support for Fn::ImportValue intrinsic.  

@see http://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/intrinsic-function-reference-importvalue.html

func.go

@@ -99,6 +99,11 @@ func unmarshalFunc(data []byte) (Func, error) {
 			if err := json.Unmarshal(data, &f); err == nil {
 				return f, nil
 			}
+		case "Fn::ImportValue":
+			f := ImportValueFunc{}
+			if err := json.Unmarshal(data, &f); err == nil {
+				return f, nil
+			}
 		default:
 			return nil, UnknownFunctionError{Name: key}
 		}

func_importvalue.go

@@ -0,0 +1,43 @@
+package cloudformation
+
+// ImportValue returns a new instance of ImportValue that imports valueToImport.
+func ImportValue(valueToImport string) ImportValueFunc {
+	return ImportValueFunc{ValueToImport: valueToImport}
+}
+
+// ImportValueFunc represents an invocation of the Fn::ImportValue intrinsic.
+// The intrinsic function Fn::ImportValue returns the value of an output exported by
+// another stack. You typically use this function to create cross-stack references.
+// In the following example template snippets, Stack A exports VPC security group
+// values and Stack B imports them.
+//
+// Note
+// The following restrictions apply to cross-stack references:
+//    For each AWS account, Export names must be unique within a region.
+//    You can't create cross-stack references across different regions. You can
+//      use the intrinsic function Fn::ImportValue only to import values that
+//      have been exported within the same region.
+//    For outputs, the value of the Name property of an Export can't use
+//      functions (Ref or GetAtt) that depend on a resource.
+//    Similarly, the ImportValue function can't include functions (Ref or GetAtt)
+//      that depend on a resource.
+//    You can't delete a stack if another stack references one of its outputs.
+//    You can't modify or remove the output value as long as it's referenced by another stack.
+//
+// See http://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/intrinsic-function-reference-importvalue.html
+type ImportValueFunc struct {
+	ValueToImport string `json:"Fn::ImportValue"`
+}
+
+// String returns this reference as a StringExpr
+func (r ImportValueFunc) String() *StringExpr {
+	return &StringExpr{Func: r}
+}
+
+// StringList returns this reference as a StringListExpr
+func (r ImportValueFunc) StringList() *StringListExpr {
+	return &StringListExpr{Func: r}
+}
+
+var _ StringFunc = ImportValueFunc{}
+var _ StringListFunc = ImportValueFunc{}

func_importvalue_test.go

@@ -0,0 +1,41 @@
+package cloudformation
+
+import (
+	"encoding/json"
+
+	. "gopkg.in/check.v1"
+)
+
+type ImportValueFuncTest struct{}
+
+var _ = Suite(&ImportValueFuncTest{})
+
+func (testSuite *ImportValueFuncTest) TestRef(c *C) {
+	inputBuf := `{"Fn::ImportValue" : "sharedValueToImport"}`
+	f, err := unmarshalFunc([]byte(inputBuf))
+	c.Assert(err, IsNil)
+	c.Assert(f.(StringFunc).String(), DeepEquals, ImportValue("sharedValueToImport").String())
+
+	// tidy the JSON input
+	inputStruct := map[string]interface{}{}
+	_ = json.Unmarshal([]byte(inputBuf), &inputStruct)
+	expectedBuf, _ := json.Marshal(inputStruct)
+
+	buf, err := json.Marshal(f)
+	c.Assert(err, IsNil)
+	c.Assert(string(buf), Equals, string(expectedBuf))
+}
+
+func (testSuite *ImportValueFuncTest) TestFailures(c *C) {
+	inputBuf := `{"Fn::ImportValue": {"Fn::ImportValue": "foo"}}`
+	_, err := unmarshalFunc([]byte(inputBuf))
+	c.Assert(err, ErrorMatches, "cannot decode function")
+
+	inputBuf = `{"Fn::ImportValue": ["1"]}`
+	_, err = unmarshalFunc([]byte(inputBuf))
+	c.Assert(err, ErrorMatches, "cannot decode function")
+
+	inputBuf = `{"Fn::ImportValue": true}`
+	_, err = unmarshalFunc([]byte(inputBuf))
+	c.Assert(err, ErrorMatches, "cannot decode function")
+}