Debuggers in Lisps Suvrat Apte (with Clojure) (Helpshift Inc) 

Outline • Why we need debuggers • Demo: Clojure debugging • How GDB works • REPL middleware • How cider-debug works • Limitations • Questions 

Debuggers • Examine state of programs as they run • Halt execution • Step through execution • Modify state at runtime • Change flow of execution 

cider-nrepl Demo Clojure debugging 

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 

GDB - GNU Debugger • Dependence on OS for process tracing • Dependence on OS for signals • Dependence on processors for interrupts GDB Target OS Processor 

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? 

Clojure debugger f (x) = y Debugger (code) = debuggable code 

What is a REPL? Read-Eval-Print-Loop (defn repl [] (let [input (read) result (eval input)] (println result) (recur))) 

Read Eval Print Loop (REPL) REPL Prompt: "=> (+ 1 2) "=> 3 "=> REPL client REPL server Operation: “eval” Code: (+ 1 2) 

Read Eval Print Loop (REPL) REPL client REPL server Operation: “eval” Result: 3 REPL Prompt: "=> (+ 1 2) "=> 3 "=> 

nREPL Middleware nREPL CIDER nREPL CIDER Middleware 

cider-nrepl •CIDER uses cider-nrepl middleware •A collection of nREPL middleware • complete • info • debug • More. . . 

cider-nrepl Demo Client-server messaging 

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})) 

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 

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 

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 

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 

That’s why you should use Emacs! 

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 

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 

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 

How does it work? • Simple eval • Reader macro: #dbg • cider.nrepl.middleware.util.instrument/walk-indexed • Assigns coordinates to forms recursively 

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 

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 

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 

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 

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 

Clojure debugger f (x) = y Debugger (code) = debuggable code 

Clojure debugger f (x) = y Debugger (code) = debuggable code Clojure Clojure 

GDB - GNU Debugger f (p, q, r) = y Debugger (code, os, processor) = debuggable code C Assembly 

Limitations • Cannot instrument bytecode • Cannot go back in time • Cannot replay 

cider-debug • Debug.clj: 655 lines • Instrument.clj: 372 lines • The code is very readable! 

cider-debug • Bozhidar Batsov • Artur Malabara • Vitalie Spinu • Chris Perkins • Neil Okamoto • Michael Griffiths • Lars Anderson • Dominic Monroe Thank you! 

Lisp is similar to Chess. It will take only a day to learn the rules. But years to master ! - Harold Abelson