Allow to set nil

el/el.go

@@ -101,8 +101,10 @@ func (i *DefaultInterpreter) Execute(
 		funcs[k] = v
 	}
 	var res interface{}
+	resultEvaluated := false
 	funcs["set"] = func(r interface{}) interface{} {
 		res = r
+		resultEvaluated = true
 		return r
 	}
 	funcs["eval"] = func(intrpr, expr string) (interface{}, error) {
@@ -135,7 +137,7 @@ func (i *DefaultInterpreter) Execute(
 	if err != nil {
 		return nil, err
 	}
-	if res != nil {
+	if resultEvaluated {
 		return res, nil
 	}
 	return buf.String(), nil

structor.go

@@ -138,7 +138,7 @@ func (ev evaluator) eval(s, extra, substruct, subctx interface{}) error {
 			if err != nil {
 				return err
 			}
-			err = reflectSet(f.value, result)
+			err = reflectSet(f.value, f.typ, result)
 			if err == nil {
 				return nil
 			}
@@ -174,6 +174,7 @@ type fieldDescr struct {
 	expr        string
 	interpreter el.Interpreter
 	value       reflect.Value
+	typ         reflect.Type
 	tags        map[string]string
 }
 
@@ -187,6 +188,7 @@ func (ev evaluator) fieldIntrospect(
 	res := fieldDescr{
 		name:  f.Name,
 		value: v,
+		typ:   f.Type,
 		tags:  tags,
 	}
 	if err != nil {
@@ -215,7 +217,7 @@ func (ev evaluator) fieldIntrospect(
 
 var errTryRecursive = errors.New("try recursive") // sentinel error
 
-func reflectSet(v reflect.Value, nv interface{}) (err error) {
+func reflectSet(v reflect.Value, vt reflect.Type, nv interface{}) (err error) {
 	defer func() {
 		if r := recover(); r != nil {
 			var ok bool
@@ -224,14 +226,17 @@ func reflectSet(v reflect.Value, nv interface{}) (err error) {
 			}
 		}
 	}()
+	if nv == nil {
+		v.Set(reflect.Zero(vt))
+		return nil
+	}
 	vnv := reflect.ValueOf(nv)
-	vt := v.Type()
 	if !vnv.Type().ConvertibleTo(vt) &&
 		v.Kind() == reflect.Struct {
 		// Try to recursively eval tags on inner struct.
 		return errTryRecursive
 	}
-	// Try to convert, in worst case it'll give a panic with suitable message.
+	// Try to convert, it may give a panic with suitable message.
 	v.Set(vnv.Convert(vt))
 	return nil
 }

structor_test.go

@@ -27,6 +27,7 @@ func TestSimple(t *testing.T) {
 		D string `eval:"{{.Struct.C}}"`
 		E string `eval:"eee"`
 		F string `eval:"{{.Struct.E}} + {{.Extra.X}}"`
+		G string `eval:""`
 	}
 	v := &simple{
 		A: "init A",
@@ -57,6 +58,7 @@ func TestSimple(t *testing.T) {
 	assert.Equal(t, "extra field X", v.D)
 	assert.Equal(t, "eee", v.E)
 	assert.Equal(t, "eee + extra field X", v.F)
+	assert.Equal(t, "", v.G)
 }
 
 // TestObj tests structor usage with non-string fields, type conversion,
@@ -83,8 +85,10 @@ func TestObj(t *testing.T) {
 			J string `eval:"jjj"`
 		}
 		K inner
+		N interface{} `eval:"{{set nil}}"`
+		P string      `eval:{{or .Val "second"}}`
 	}
-	v := &obj{F: &innerSub{}}
+	v := &obj{F: &innerSub{}, N: "xxx", P: "first"}
 	ev := structor.NewDefaultEvaluator(use.Packages(
 		use.Pkg{Prefix: "", Funcs: math.Pkg},
 		use.Pkg{Prefix: "", Funcs: encoding.Pkg},
@@ -103,6 +107,8 @@ func TestObj(t *testing.T) {
 	assert.Equal(t, "\n", v.H)
 	assert.Equal(t, "jjj", v.I.J)
 	assert.Equal(t, "LLL", v.K.L)
+	assert.Nil(t, v.N)
+	assert.Equal(t, "first", v.P)
 }
 
 // TestError tests structor's error handling.