Gore: A Tale of Go REPL

Transcript

1. A Tale of Go REPL motemen Go Conference 2015 Summer

   @ Tokyo

2. About Me - @motemen - Web engineer - Chief engineer

   at Hatena, Kyoto

3. Mackerel mackerel.io

4. Agenda - Introduction of ! motemen/gore - Gore’s architecture -

   Implementation details

5. Gore /gɔː/

6. Case: Learning New Languages - Playing with language features -

   Trying libraries

7. My Case — Perl - perl ~/sketch.pl - Edit and

   Run

8. My Case — Ruby - irb - pry - A

   “REPL”
 (Read-Eval-Print-Loop)

9. My Case — Go? - tmp.go - “packge main” “func

   main()” - Removing unused variables by hand - Question: Can gophers haz a REPL?

10. Go "REPL" impls (AFAIK) - ! sbinet/igo — built upon

    ! sbinet/go-eval, with liner - ! emicklei/rango — importing, statements, listing variables - ! vito/go-repl — importing, statements, useful commands - ! d4l3k/go-pry — attaches to running program, uses reflection - (play.golang.org)

11. Needed a REPL that: - Evaluates statements - Imports any

    module - Pretty prints values automatically - Completes code

12. The Difficulties - How can we achieve Go semantics? -

    No APIs provided - Re-implement Go compiler?

13. Solution: “go run”

14. Solution: “go run” - An “eval” to Go - Perfect

    implementation & Always up-to-date - Dead simple - In: a program Out: the result

15. Gore: A REPL using “go run” go get github.com/motemen/gore

16. Caveats - Gore runs all code input for each run

    - Code with side effects will be repeated - eg. Printing output / Sleeping

17. Stories Inside Gore

18. Overview: not a “REPL” exactly - REPL: Read-Eval-Print-Loop - No

    “Eval” and “Print” phase - As we do them by “go run” - Much like: Read-Generate-Run-Loop

19. Read-Gen-Run-Loop 1. Read user input to evaluate 2. Generate a

    .go file that prints result 3. “go run” it 4. Back to 1.

20. Step: Read - Codes go under AST (abstract syntax tree)

    form - Syntax checking - Easier to manipulate - Input String → AST → .go → “go run”

21. Read: Input - ! peterh/liner - Line editing - History

    - Supports Windows

22. Read: Input to AST - package go/parser - Go has

    rich support for parsing/typing its code - Easy to generate & manipulate

23. Read: Input to AST - Parse input as an expression

    - If failed: a statement - Otherwise: continue input (multi-line)

24. Read: Expression - Easy - Dedicated API - parser.ParseExpr(in)

25. Read: Statements - $ func (s *Session) evalStmt(in string) error

    { src := fmt.Sprintf("package P; func F() { %s }", in) f, err := parser.ParseFile(s.Fset, "stmt.go", src, 0) if err != nil { return err } enclosingFunc := f.Scope.Lookup("F").Decl. (*ast.FuncDecl) stmts := enclosingFunc.Body.List }

26. Generate: The Delicious Part - Append the input AST nodes

    to our main() - Add the code to print the value

27. Printing Values - The generated program prints values (not gore)

    - “New Values” should be printed - Expression: Its resulting value - Statement: Values assigned

28. Printing Values: Expression - User: foo.Bar(a+1) - Gore: __gore_p(foo.Bar(a+1))

29. Printing Values: Statements - ast.AssignStmt stands for assign/define - User:

    a, b := foo() - Gore: a, b := foo(); __gore_p(a, b)

30. Printing Values: __gore_p() - “Pretty prints” values - Defined along

    with the main() - Depends on installed packages - ! k0kubun/pp, ! davecgh/go-spew or plain old %#v

31. The Initial .go File (pp) package main import "github.com/k0kubun/pp" func

    __gore_p(xx ...interface{}) { for _, x := range xx { pp.Println(x) } } func main() { // User input goes here }

32. The Initial .go File (go-spew) package main import "github.com/davecgh/go-spew/spew" func

    __gore_p(xx ...interface{}) { for _, x := range xx { spew.Printf("%#v\n", x) } } func main() { // User input goes here }

33. The Initial .go File (%#v) package main import "fmt" func

    __gore_p(xx ...interface{}) { for _, x := range xx { fmt.Printf("%#v\n", x) } } func main() { // User input goes here }

34. Generate: Append to main() - Just append those generated statements


    s.mainBody.List = append(s.mainBody.List, stmts…) - Now we’ve got the working code! - Really? No! %

35. Go compiler complaints (you know) - “x declared but not

    used” - “p imported but not used” - “no new variables of left side of :=“ - & Should remove these to a successful go run

36. Quick Fixing Erroneous Program - “x declared but not used”

    - “p imported but not used” - “no new variables on left side of :=“ Use it!! Anonymize it!! Make it `=‘ !!

37. “declared but not used” package P func main() { s

    := "hello" } package P func main() { s := "hello" _ = s }

38. ”imported but not used” package P import "fmt" func main()

    { } package P import _ "fmt" func main() { }

39. “no new variables on left side of :=“ package P

    func main() { var a int a := 1 } package P func main() { var a int a = 1 }

40. ! motemen/go-quickfix to do the work - Type check source

    code using go/types.Check() - Catch errors and modify AST - Packed with a bin - goquickfix -w main.go

41. Gore specific quickfix - __gore_p() for non-value expressions

42. Running - Output the AST to file — go/printer -

    Then go run - If failed to run, revert the last input

43. Recap: Read-Gen-Quickfix-Run-Loop 1. Read and parse the input to obtain

    AST 2. Add printing function call __gore_p() 3. Append the code to main() 4. Quickfix it so that it compiles well 5. go run

44. PRs welcome! - ! motemen/gore - ! motemen/go-quickfix

45. htn.to/intern2015

46. A Tale Gore motemen Go Conference 2015 Summer @ Tokyo

47. Appendix: More Features

48. Code Completion - liner has support for completion - Great

    tool for editors: ! nsf/gocode - Server/client model - And an interface to it: motemen/gore/gocode - If gocode binary can be located, use it

49. Commands - :print - :write <file> - :import <pkg> -

    :doc <expr>

50. :import <pkg> - Imports arbitrary packages - Just append import

    statement to the file - Unused imports are handled by quickfix

51. :doc <expr> - Invokes go doc for given expression -

    Package: :doc json - Function: :doc json.Marshal - Method: :doc json.NewEncoder().Encode