starlark/types: ref function

starlark/runtime/runtime.go

@@ -58,6 +58,7 @@ func NewRuntime(pm *terraform.PluginManager) *Runtime {
 	predeclared["validate"] = types.BuiltinValidate()
 	predeclared["hcl"] = types.BuiltinHCL()
 	predeclared["fn"] = types.BuiltinFunctionAttribute()
+	predeclared["ref"] = types.BuiltinRef()
 	predeclared["evaluate"] = types.BuiltinEvaluate(predeclared)
 	predeclared["struct"] = starlark.NewBuiltin("struct", starlarkstruct.Make)
 	predeclared["module"] = starlark.NewBuiltin("module", starlarkstruct.MakeModule)

starlark/types/attribute.go

@@ -10,6 +10,40 @@ import (
 // sTring alias required to avoid name collision with the method String.
 type sString = starlark.String
 
+// BuiltinRef returns a starlak.Builtin function to generate a reference to a
+// resource argument.
+//
+//   outline: types
+//     functions:
+//       ref(resource, argument) string
+//         Returns a reference to a resource argument.
+//         params:
+//           resource <resource>
+//             resource to be referenced.
+//           field string
+//             field to be referenced.
+//
+//         examples:
+//           ref.star
+//
+func BuiltinRef() starlark.Value {
+	return starlark.NewBuiltin("ref", func(_ *starlark.Thread, _ *starlark.Builtin, args starlark.Tuple, kwargs []starlark.Tuple) (starlark.Value, error) {
+		var resource *Resource
+		var argument string
+		err := starlark.UnpackArgs("ref", args, kwargs, "resource", &resource, "argument", &argument)
+		if err != nil {
+			return nil, err
+		}
+
+		attr, ok := resource.block.Attributes[argument]
+		if !ok {
+			return nil, fmt.Errorf("%s has no .%s field", resource, argument)
+		}
+
+		return NewAttribute(resource, attr.Type, argument), nil
+	})
+}
+
 // Attribute is a reference to an argument of a Resource. Used mainly
 // for Computed arguments of Resources.
 //

starlark/types/provider_test.go

@@ -83,6 +83,7 @@ func doTestPrint(t *testing.T, filename string, print func(*starlark.Thread, str
 	predeclared["hcl"] = BuiltinHCL()
 	predeclared["validate"] = BuiltinValidate()
 	predeclared["fn"] = BuiltinFunctionAttribute()
+	predeclared["ref"] = BuiltinRef()
 	predeclared["evaluate"] = BuiltinEvaluate(predeclared)
 	predeclared["struct"] = starlark.NewBuiltin("struct", starlarkstruct.Make)
 	predeclared["module"] = starlark.NewBuiltin("module", starlarkstruct.MakeModule)

starlark/types/testdata/attribute.star

@@ -56,3 +56,13 @@ k8s = tf.provider("kubernetes")
 secret = k8s.data.secret("foo")
 assert.eq(str(secret.data["qux"]), "${data.kubernetes_secret.foo.data.qux}")
 assert.eq(str(secret.data["qux"][0]), "${data.kubernetes_secret.foo.data.qux.0}")
+
+# ref
+secret = k8s.resource.secret("foo")
+secret.metadata.name = "foo"
+secret.type = "kubernetes.io/dockerconfigjson"
+assert.eq(str(ref(secret, "type")), "${kubernetes_secret.foo.type}")
+assert.eq(str(ref(secret.metadata, "name")), "${kubernetes_secret.foo.metadata.0.name}")
+
+# ref to non-exits
+assert.fails(lambda: ref(secret, "foo"), "Resource<kubernetes.resource.kubernetes_secret> has no .foo field")

starlark/types/testdata/examples/ref.star

@@ -0,0 +1,12 @@
+# Non-computed arguments are passed by value to one argument to another 
+# in some cases may be required to make a referece to and relay in the HCL
+# interpolation. 
+
+aws = tf.provider("aws")
+
+instance = aws.resource.instance("foo")
+instance.ami = "foo"
+
+print(ref(instance, "ami"))
+# Output:
+# ${aws_instance.foo.ami}