diff --git a/config/interpolate_funcs.go b/config/interpolate_funcs.go
index c67e3b733c..8f19d3b95f 100644
--- a/config/interpolate_funcs.go
+++ b/config/interpolate_funcs.go
@@ -15,9 +15,8 @@ var Funcs map[string]lang.Function
 
 func init() {
 	Funcs = map[string]lang.Function{
-		"file": interpolationFuncFile(),
-		"join": interpolationFuncJoin(),
-		//"lookup":  interpolationFuncLookup(),
+		"file":    interpolationFuncFile(),
+		"join":    interpolationFuncJoin(),
 		"element": interpolationFuncElement(),
 	}
 }
@@ -59,22 +58,22 @@ func interpolationFuncJoin() lang.Function {
 
 // interpolationFuncLookup implements the "lookup" function that allows
 // dynamic lookups of map types within a Terraform configuration.
-func interpolationFuncLookup(
-	vs map[string]string, args ...string) (string, error) {
-	if len(args) != 2 {
-		return "", fmt.Errorf(
-			"lookup expects 2 arguments, got %d", len(args))
-	}
+func interpolationFuncLookup(vs map[string]string) lang.Function {
+	return lang.Function{
+		ArgTypes:   []ast.Type{ast.TypeString, ast.TypeString},
+		ReturnType: ast.TypeString,
+		Callback: func(args []interface{}) (interface{}, error) {
+			k := fmt.Sprintf("var.%s.%s", args[0].(string), args[1].(string))
+			v, ok := vs[k]
+			if !ok {
+				return "", fmt.Errorf(
+					"lookup in '%s' failed to find '%s'",
+					args[0].(string), args[1].(string))
+			}
 
-	k := fmt.Sprintf("var.%s", strings.Join(args, "."))
-	v, ok := vs[k]
-	if !ok {
-		return "", fmt.Errorf(
-			"lookup in '%s' failed to find '%s'",
-			args[0], args[1])
+			return v, nil
+		},
 	}
-
-	return v, nil
 }
 
 // interpolationFuncElement implements the "element" function that allows
diff --git a/config/interpolate_funcs_test.go b/config/interpolate_funcs_test.go
index 3a9a86fe14..9ea2ac1cb7 100644
--- a/config/interpolate_funcs_test.go
+++ b/config/interpolate_funcs_test.go
@@ -20,163 +20,149 @@ func TestInterpolateFuncFile(t *testing.T) {
 	tf.Close()
 	defer os.Remove(path)
 
-	testFunction(t, []testFunctionCase{
-		{
-			fmt.Sprintf(`${file("%s")}`, path),
-			"foo",
-			false,
-		},
+	testFunction(t, testFunctionConfig{
+		Cases: []testFunctionCase{
+			{
+				fmt.Sprintf(`${file("%s")}`, path),
+				"foo",
+				false,
+			},
 
-		// Invalid path
-		{
-			`${file("/i/dont/exist")}`,
-			nil,
-			true,
-		},
+			// Invalid path
+			{
+				`${file("/i/dont/exist")}`,
+				nil,
+				true,
+			},
 
-		// Too many args
-		{
-			`${file("foo", "bar")}`,
-			nil,
-			true,
+			// Too many args
+			{
+				`${file("foo", "bar")}`,
+				nil,
+				true,
+			},
 		},
 	})
 }
 
 func TestInterpolateFuncJoin(t *testing.T) {
-	testFunction(t, []testFunctionCase{
-		{
-			`${join(",")}`,
-			nil,
-			true,
-		},
-
-		{
-			`${join(",", "foo")}`,
-			"foo",
-			false,
-		},
-
-		/*
-			TODO
+	testFunction(t, testFunctionConfig{
+		Cases: []testFunctionCase{
 			{
-				`${join(",", "foo", "bar")}`,
-				"foo,bar",
+				`${join(",")}`,
+				nil,
+				true,
+			},
+
+			{
+				`${join(",", "foo")}`,
+				"foo",
 				false,
 			},
-		*/
 
-		{
-			fmt.Sprintf(`${join(".", "%s")}`,
-				fmt.Sprintf(
-					"foo%sbar%sbaz",
-					InterpSplitDelim,
-					InterpSplitDelim)),
-			"foo.bar.baz",
-			false,
+			/*
+				TODO
+				{
+					`${join(",", "foo", "bar")}`,
+					"foo,bar",
+					false,
+				},
+			*/
+
+			{
+				fmt.Sprintf(`${join(".", "%s")}`,
+					fmt.Sprintf(
+						"foo%sbar%sbaz",
+						InterpSplitDelim,
+						InterpSplitDelim)),
+				"foo.bar.baz",
+				false,
+			},
 		},
 	})
 }
 
-/*
 func TestInterpolateFuncLookup(t *testing.T) {
-	testFunction(t, []testFunctionCase{
-	cases := []struct {
-		M      map[string]string
-		Args   []string
-		Result string
-		Error  bool
-	}{
-		{
-			map[string]string{
-				"var.foo.bar": "baz",
+	testFunction(t, testFunctionConfig{
+		Vars: map[string]string{"var.foo.bar": "baz"},
+		Cases: []testFunctionCase{
+			{
+				`${lookup("foo", "bar")}`,
+				"baz",
+				false,
 			},
-			[]string{"foo", "bar"},
-			"baz",
-			false,
-		},
 
-		// Invalid key
-		{
-			map[string]string{
-				"var.foo.bar": "baz",
+			// Invalid key
+			{
+				`${lookup("foo", "baz")}`,
+				nil,
+				true,
 			},
-			[]string{"foo", "baz"},
-			"",
-			true,
-		},
 
-		// Too many args
-		{
-			map[string]string{
-				"var.foo.bar": "baz",
+			// Too many args
+			{
+				`${lookup("foo", "bar", "baz")}`,
+				nil,
+				true,
 			},
-			[]string{"foo", "bar", "baz"},
-			"",
-			true,
 		},
-	}
-
-	for i, tc := range cases {
-		actual, err := interpolationFuncLookup(tc.M, tc.Args...)
-		if (err != nil) != tc.Error {
-			t.Fatalf("%d: err: %s", i, err)
-		}
-
-		if actual != tc.Result {
-			t.Fatalf("%d: bad: %#v", i, actual)
-		}
-	}
+	})
 }
-*/
 
 func TestInterpolateFuncElement(t *testing.T) {
-	testFunction(t, []testFunctionCase{
-		{
-			fmt.Sprintf(`${element("%s", "1")}`,
-				"foo"+InterpSplitDelim+"baz"),
-			"baz",
-			false,
-		},
+	testFunction(t, testFunctionConfig{
+		Cases: []testFunctionCase{
+			{
+				fmt.Sprintf(`${element("%s", "1")}`,
+					"foo"+InterpSplitDelim+"baz"),
+				"baz",
+				false,
+			},
 
-		{
-			`${element("foo", "0")}`,
-			"foo",
-			false,
-		},
+			{
+				`${element("foo", "0")}`,
+				"foo",
+				false,
+			},
 
-		// Invalid index should wrap vs. out-of-bounds
-		{
-			fmt.Sprintf(`${element("%s", "2")}`,
-				"foo"+InterpSplitDelim+"baz"),
-			"foo",
-			false,
-		},
+			// Invalid index should wrap vs. out-of-bounds
+			{
+				fmt.Sprintf(`${element("%s", "2")}`,
+					"foo"+InterpSplitDelim+"baz"),
+				"foo",
+				false,
+			},
 
-		// Too many args
-		{
-			fmt.Sprintf(`${element("%s", "0", "2")}`,
-				"foo"+InterpSplitDelim+"baz"),
-			nil,
-			true,
+			// Too many args
+			{
+				fmt.Sprintf(`${element("%s", "0", "2")}`,
+					"foo"+InterpSplitDelim+"baz"),
+				nil,
+				true,
+			},
 		},
 	})
 }
 
+type testFunctionConfig struct {
+	Cases []testFunctionCase
+	Vars  map[string]string
+}
+
 type testFunctionCase struct {
 	Input  string
 	Result interface{}
 	Error  bool
 }
 
-func testFunction(t *testing.T, cases []testFunctionCase) {
-	for i, tc := range cases {
+func testFunction(t *testing.T, config testFunctionConfig) {
+	for i, tc := range config.Cases {
 		ast, err := lang.Parse(tc.Input)
 		if err != nil {
 			t.Fatalf("%d: err: %s", i, err)
 		}
 
-		engine := langEngine(nil)
+		engine := langEngine(config.Vars)
 		out, _, err := engine.Execute(ast)
 		if (err != nil) != tc.Error {
 			t.Fatalf("%d: err: %s", i, err)
diff --git a/config/raw_config.go b/config/raw_config.go
index 37867c638a..0e19391849 100644
--- a/config/raw_config.go
+++ b/config/raw_config.go
@@ -208,10 +208,17 @@ func langEngine(vs map[string]string) *lang.Engine {
 	for k, v := range vs {
 		varMap[k] = lang.Variable{Value: v, Type: ast.TypeString}
 	}
+
+	funcMap := make(map[string]lang.Function)
+	for k, v := range Funcs {
+		funcMap[k] = v
+	}
+	funcMap["lookup"] = interpolationFuncLookup(vs)
+
 	return &lang.Engine{
 		GlobalScope: &lang.Scope{
 			VarMap:  varMap,
-			FuncMap: Funcs,
+			FuncMap: funcMap,
 		},
 	}
 }