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Expression language(EL) to navigate/manipulate in golang structure data
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.gitignore add parser tool Nov 7, 2015
expression_test.go Fixes #1 Nov 11, 2015
lexer.go refactor Nov 9, 2015
patcher.go memory waste when extend slice Nov 16, 2015


Expression language(EL) to manipulate Golang structure data. Its main purpose is to find reflect.Value by Expression, then do some reading and writing.


Simple as it takes to type the following command:

go get

and import with



Example Data

As example, we have some data like this:

type Comment struct {
	NickName string
	Content  string
	Date     time.Time

type Author struct {
  Name string

type Blog struct {
	Title      string
	RoleState  map[string]uint
	CommentIds []uint64
	Comments   map[string]*Comment

func (b Blog) FirstComment() *Comment {
	return b.Comments["0"]

then we init them with some test data:

b := &Blog{
  Title:      "Blog title1",
  CommentIds: []uint64{1, 3},
  Comments: map[string]*Comment{
    "0": {
      NickName: "000",
      Content:  "test",
      Date:     time.Now(),
    "1": {
      NickName: "u1",
      Content:  "test",
      Date:     time.Now(),
    "3": {
      NickName: "tester",
      Content:  "test hehe...",
      Date:     time.Now(),
  Author: Author{
      Name: "Author 1",
  RoleState: map[string]uint{},


Using el.Expression, we can navigate from root(b) to anywhere in this structure.

1. To field

exp := el.Expression("Title")
v, _ := exp.Execute(&data)
fmt.Printf("%v\n", v.interface()) //==> Blog title1

2. To nested field

exp := el.Expression("Author.Name")
v, _ := exp.Execute(&data)
fmt.Printf("%v\n", v.interface()) //==> Author 1

3. To slice/array/string item

exp := el.Expression("CommentIds[0]")
v, _ := exp.Execute(&data)
fmt.Printf("%v\n", v.interface()) //==> 1

4. To map item

exp := el.Expression("Comments["3"].NickName")
v, _ := exp.Execute(&data)
fmt.Printf("%v\n", v.interface()) //==> tester

5. Item in[] also can be another Expression

exp := el.Expression("Comments["CommentIds[0]].NickName")
v, _ := exp.Execute(&data)
fmt.Printf("%v\n", v.interface()) //==> u1

6. Call function

function can return only ONE result

exp := el.Expression("FirstComment().Content")
v, _ := exp.Execute(&data)
fmt.Printf("%v\n", v.interface()) //==> test  

7. Modify Value

After Execute expression, we got a relfect.Value, we also can use it to modify data, e.g.

exp := el.Expression("FirstComment().Content")
v, _ := exp.Execute(&data)

will let first comment with value 1111

Beside that we recommend users take a moment to look The Laws of Reflection, take care some limition that reflect has.


Base on Expression, we also provide a tool named Patcher, the purpose of it is to let use modify object with expression easier and be batched.

We found it's very useful to build HTTP Patch API to partial update entity

ps := p.Patch{
  "Author.Name":                      "ほん",
  "Comments[CommentIds[0]].NickName": "私",
  "roleState[100]":                   uint(100),
err := patcher.PatchIt(b, ps)

This will modify three properties at once~ (but we still meet some rule of refect, like map-value use ptr.. and so on)


See our Example in Unit-Test:


generate expression between two diff..- -?


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