Gore: A Tale of Go REPL

Go Conference Summer 2015

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