Debuggers in Lispy Languages

This talk is about a unique way in which debuggers can be written in language which are lisp like.
With CIDER debug as an example, this talk takes you through how this approach works under the hood.
This talk was presented at Functional Conf 2019.

Transcript

1. Debuggers in Lisps Suvrat Apte (with Clojure) (Helpshift Inc)
2. Outline • Why we need debuggers • Demo: Clojure debugging • How GDB works • REPL middleware • How cider-debug works • Limitations • Questions
3. Debuggers • Examine state of programs as they run • Halt execution • Step through execution • Modify state at runtime • Change flow of execution
4. cider-nrepl Demo Clojure debugging
5. GDB - GNU Debugger • Debugger runs as a separate process • Spawns the target program as a child • Examines state with process tracing calls • Implementing breakpoints is a trick! • Puts invalid instructions on breakpoints • Processor traps OS • OS signals the process which caused it • It passes the signal to its parent (GDB) • GDB checks IP and breaks • Some processors support breakpoints GDB Target OS Processor
6. GDB - GNU Debugger • Dependence on OS for process tracing • Dependence on OS for signals • Dependence on processors for interrupts GDB Target OS Processor
7. Lisps • Lisp has minimal syntax • Code is data • Code itself is a valid AST • Debuggers can use code as data (map sq-root (filter even? numbers)) map sq-root filter numbers even?
8. Clojure debugger f (x) = y Debugger (code) = debuggable code
9. What is a REPL? Read-Eval-Print-Loop (defn repl [] (let [input (read) result (eval input)] (println result) (recur)))
10. Read Eval Print Loop (REPL) REPL Prompt: "=> (+ 1 2) "=> 3 "=> REPL client REPL server Operation: “eval” Code: (+ 1 2)
11. Read Eval Print Loop (REPL) REPL client REPL server Operation: “eval” Result: 3 REPL Prompt: "=> (+ 1 2) "=> 3 "=>
12. nREPL Middleware nREPL CIDER nREPL CIDER Middleware
13. cider-nrepl •CIDER uses cider-nrepl middleware •A collection of nREPL middleware • complete • info • debug • More. . .
14. cider-nrepl Demo Client-server messaging
15. Form based coords Coor: (3 1 1 1) (defn math-operations [n1 n2] (let [addition (+ n1 n2) subtraction (- n1 n2) exp (power n1 n2)] {:addition addition :subtraction subtraction :exp exp}))
16. Coor: (3 1 1 1) (defn math-operations [n1 n2] (let [addition (+ n1 n2) subtraction (- n1 n2) exp (power n1 n2)] {:addition addition :subtraction subtraction :exp exp})) 0 1 2 3 Form based coords
17. Coor: (3 1 1 1) (defn math-operations [n1 n2] (let [addition (+ n1 n2) subtraction (- n1 n2) exp (power n1 n2)] {:addition addition :subtraction subtraction :exp exp})) 0 1 2 Form based coords
18. Coor: (3 1 1 1) (defn math-operations [n1 n2] (let [addition (+ n1 n2) subtraction (- n1 n2) exp (power n1 n2)] {:addition addition :subtraction subtraction :exp exp})) 0 1 2 3 4 5 Form based coords
19. Coor: (3 1 1 1) (defn math-operations [n1 n2] (let [addition (+ n1 n2) subtraction (- n1 n2) exp (power n1 n2)] {:addition addition :subtraction subtraction :exp exp})) 0 1 2 Form based coords
20. That’s why you should use Emacs!
21. How does it work? 1. Walk through code 1. Mark forms/symbols with coordinates 2. Expand macros 3. Walk through code 1. Wrap forms/symbols with pre-breakpoint macro 4. Pre-breakpoint macro expansion 1. Put a breakpoint if necessary When a function is marked for debugging
22. How does it work? 1. Necessary information is sent to client 1. Coordinates 2. Return value at current position 3. Values of local vars 4. Possible actions 2. Waits for response from client. . . When a breakpoint is hit
23. How does it work? 1. Walk through code 1. Mark forms/symbols with coordinates 2. Expand macros 3. Walk through code 1. Wrap forms/symbols with pre-breakpoint macro 4. Pre-breakpoint macro expansion 1. Put a breakpoint if necessary When a function is marked for debugging
24. How does it work? • Simple eval • Reader macro: #dbg • cider.nrepl.middleware.util.instrument/walk-indexed • Assigns coordinates to forms recursively
25. How does it work? 1. Walk through code 1. Mark forms/symbols with coordinates 2. Expand macros 3. Walk through code 1. Wrap forms/symbols with pre-breakpoint macro 4. Pre-breakpoint macro expansion 1. Put a breakpoint if necessary When a function is marked for debugging
26. How does it work? 1. Walk through code 1. Mark forms/symbols with coordinates 2. Expand macros 3. Walk through code 1. Wrap forms/symbols with pre-breakpoint macro 4. Pre-breakpoint macro expansion 1. Put a breakpoint if necessary When a function is marked for debugging
27. How does it work? 1. Walk through code 1. Mark forms/symbols with coordinates 2. Expand macros 3. Walk through code 1. Wrap forms/symbols with pre-breakpoint macro 4. Pre-breakpoint macro expansion 1. Put a breakpoint if necessary When a function is marked for debugging
28. How does it work? 1. Necessary information is sent to client 1. Coordinates 2. Return value at current position 3. Values of local vars 4. Possible actions 2. Waits for response from client. . . When a breakpoint is hit
29. How does it work? 1. Necessary information is sent to client 1. Coordinates 2. Return value at current position 3. Values of local vars 4. Possible actions 2. Waits for response from client. . . When a breakpoint is hit
30. Clojure debugger f (x) = y Debugger (code) = debuggable code
31. Clojure debugger f (x) = y Debugger (code) = debuggable code Clojure Clojure
32. GDB - GNU Debugger f (p, q, r) = y Debugger (code, os, processor) = debuggable code C Assembly
33. Limitations • Cannot instrument bytecode • Cannot go back in time • Cannot replay
34. cider-debug • Debug.clj: 655 lines • Instrument.clj: 372 lines • The code is very readable!
35. cider-debug • Bozhidar Batsov • Artur Malabara • Vitalie Spinu • Chris Perkins • Neil Okamoto • Michael Griffiths • Lars Anderson • Dominic Monroe Thank you!
36. Lisp is similar to Chess. It will take only a day to learn the rules. But years to master ! - Harold Abelson