DSL and code generation

1. Happy Programming with Go Compiler
GTG 12
poga

3. Today
• Tricks to write DSL in Go
• code generation with GO GENERATE

4. DSL in Go
s.Config(
LogLevel(WARN),
Backend(S3{
Key: "API_KEY",
Secret: "SUPER SECRET",
}),
AfterComplete(func() {
fmt.Println("service ready")
}),
)
Do(With{A: "foo", B: 123})

5. Ruby
• DSL!
• Slow

6. Ruby
• DSL!
• Slow
• Error-prone

7. DSL in Ruby

8. DSL in Ruby
validates :name, presence: true
validate :name, presence: true
validates :name, presense: true
Which is correct?

9. DSL in Ruby
validates :points, numericality: true
validates :points, numericality: { only: “integer” }
validates :points, numericality: { only_integer: true }
Which is correct?

10. DSL in Ruby
• String/symbol + Hash everywhere
• lack of “Syntax Error”
• everything is void*
• anything is ok for ruby interpreter
• Need a lot of GOOD documentation

11. DSL in Ruby
• String/symbol + Hash everywhere
• lack of “Syntax Error”
• everything is void*
• anything is ok for ruby interpreter
• Need a lot of GOOD documentation
• and memorization

12. Go
• Fast
• Small Footprint

13. Go
• Fast
• Small Footprint
• Verbose
• sort
• if err != nil
• if err != nil
• if err != nil

14. Patterns
• http://www.godesignpatterns.com/
• https://blog.golang.org/errors-are-values

15. Named Parameter
type With struct {
A string
B int
}
func main() {
Do(With{A: "foo", B: 123})
}
func Do(p With) {
…
}

16. Named Parameter
• Error when
• mistype parameter name
• incorrect parameter type

17. Configuration
s.Config(
LogLevel(WARN),
Backend(S3{
Key: "API_KEY",
Secret: "SUPER SECRET",
}),
AfterComplete(func() {
fmt.Println("service ready")
}),
)
type Service struct {
ErrorLevel int
AWSKey string
AWSSecret string
AfterHook func()
}
https://gist.github.com/poga/bf0534486199c8e778cb

18. Configuration
• Self-referential Function
type option func(*Service)
func (s *Service) Config(opts ...option) {
for _, opt := range opts {
opt(s)
}
}
func LogLevel(level int) option {
return func(s *Service) {
s.ErrorLevel = level
}
}
http://commandcenter.blogspot.com.au/2014/01/self-referential-functions-and-design.html

19. Struct Tag
type Person struct {
FirstName string `json:"first_name"`
LastName string `json:"last_name"`
MiddleName string `json:"middle_name,omitempty"`
}

20. Struct Tag
• Anti-Pattern
• String-typing
• No compile time checking
• 6 month old bug in KKTIX production, no one noticed
• based on reflect
• slow
• Don’t use it as DSL

21. Vanilla Go
• Sometimes it’s not enough
• We need to go deeper

22. Code Generation
WOW
So Rails
Much Magic

23. Error Handling in Go
• Extremely Verbose
• We already have 3 official error handling patterns
• https://blog.golang.org/errors-are-values
• checking error is fine. But I’m lazy to type
• Messy control flow

24. Error Handler Generation
• go get github.com/poga/autoerror
• just a demo, don’t use it in production

25. Auto Error
• Add error checking function
based on the name of error
variable
• Hide error checking from
control flow

26. //go:generate autoerror -i simple.go -t AutoError -o test.go
type AutoError error
var e AutoError
var e2 AutoError
func Test() {
e = doSomething()
e2 = doSomething()
}
func doSomething() error {
return errors.New("an error")
}
func handleError2(e2 error) {
if e2 != nil {
panic(e)
}
}
func handleError(e error) {
if e != nil {
panic(e)
}
}

27. //go:generate autoerror -i simple.go -t AutoError -o test.go
type AutoError error
var e AutoError
var e2 AutoError
func Test() {
e = doSomething()
e2 = doSomething()
}
func doSomething() error {
return errors.New("an error")
}
func handleError2(e2 error) {
if e2 != nil {
panic(e)
}
}
func handleError(e error) {
if e != nil {
panic(e)
}
}

28. //go:generate autoerror -i simple.go -t AutoError -o test.go
type AutoError error
var e AutoError
var e2 AutoError
func Test() {
e = doSomething()
e2 = doSomething()
handleError2(e2)
}
func doSomething() error {
return errors.New("an error")
}
func handleError2(e2 error) {
if e2 != nil {
panic(e)
}
}
func handleError(e error) {
if e != nil {
panic(e)
}
}

29. //go:generate autoerror -i simple.go -t AutoError -o test.go
type AutoError error
var e AutoError
var e2 AutoError
func Test() {
e = doSomething()
e2 = doSomething()
handleError2(e2)
}
func doSomething() error {
return errors.New("an error")
}
func handleError2(e2 error) {
if e2 != nil {
panic(e)
}
}
func handleError(e error) {
if e != nil {
panic(e)
}
}

30. //go:generate autoerror -i simple.go -t AutoError -o test.go
type AutoError error
var e AutoError
var e2 AutoError
func Test() {
e = doSomething()
handleError(e)
e2 = doSomething()
handleError2(e2)
}
func doSomething() error {
return errors.New("an error")
}
func handleError2(e2 error) {
if e2 != nil {
panic(e)
}
}
func handleError(e error) {
if e != nil {
panic(e)
}
}

31. What is Go Generate
• shell scripting in go
$ go generate
//go:generate echo 'hello from go generate'
hello from go generate

32. go generate
• execute a ruby script
• integrate with code gen tools
• yacc, protocol buffer,… etc
//go:generate ruby gen.rb
//go:generate codecgen -o values.generated.go file1.go file2.go file3.go

33. go generate
• go get won’t execute ‘go generate’ for you
• generate before publish

34. type checking placeholder
json.have(
pair{Key: “FirstName”, From: “first_name”},
pair{Key: “FirstName”, From: “first_name”},
pair{Key: “MiddleName”, From: “middle_name”, OmitEmpty: true})
type Person struct {
FirstName string `json:"first_name"`
LastName string `json:"last_name"`
MiddleName string `json:"middle_name,omitempty"`
}

35. person.have(
string{Key: “FirstName”, From: “first_name”},
string{Key: “FirstName”, From: “first_name”},
string{Key: “MiddleName”, From: “middle_name”, OmitEmpty: true})
type Person struct {
FirstName string `json:"first_name"`
LastName string `json:"last_name"`
MiddleName string `json:"middle_name,omitempty"`
}
func (p Person) have(pairs pair…) {
panic(“run go generate before use this file”)
}

36. Recap
• DSL
• Named Parameter
• Configuration
• Code Generation
• type checking placeholder

37. Thank you