Scope Capture

scope-capture

Capture local scope at runtime and recreate it in the REPL for debugging.

Quick Start

(require 'sc.api)

;; 1. Add spy to capture scope
(defn my-fn [x y]
  (let [result (+ x y)]
    (sc.api/spy result)))  ; Captures x, y, result

;; 2. Call the function - spy logs EP id
(my-fn 3 4)
; SPY [1 -1] ... saved scope with locals [x y result]
; => 7

;; 3. Recreate scope with defsc (defs all locals as vars)
(sc.api/defsc 1)
; => [#'user/x #'user/y #'user/result]

;; 4. Now evaluate expressions using captured values
x        ; => 3
y        ; => 4
result   ; => 7
(* x y)  ; => 12

;; 5. Clean up when done
(sc.api/dispose! 1)

Core Concepts

Code Site (CS) - A location in code where spy or brk is placed. Has a negative ID like -1, -2.

Execution Point (EP) - A specific execution of a Code Site. Has a positive ID like 1, 2, 3.

Log format: SPY [EP CS] e.g., SPY [7 -3] means Execution Point 7 at Code Site -3.

Recording Scope

spy - Capture and continue

;; Wrap an expression - captures scope AND the expression value
(sc.api/spy (some-expression))

;; Without expression - just capture scope at this point
(sc.api/spy)

;; With options
(sc.api/spy {:sc/dynamic-vars [*out* *my-var*]}
  (some-expression))

brk - Capture and block (breakpoint)

(defn process [data]
  (let [transformed (transform data)]
    (sc.api/brk transformed)))  ; Blocks here until released

;; In another thread, call the function
(future (process my-data))
; BRK [2 -1] ... saved scope, use sc.api/loose to resume

;; Release the breakpoint
(sc.api/loose 2)              ; Continue normally
(sc.api/loose-with 2 value)   ; Continue with replacement value
(sc.api/loose-with-err 2 ex)  ; Continue by throwing exception

Recreating Scope

defsc - Define vars (preferred for editor eval)

(sc.api/defsc 1)
; Defines vars for all captured locals in current namespace
; Now you can evaluate sub-expressions directly in your editor

x        ; works
(+ x y)  ; works

letsc - Let bindings

(sc.api/letsc 1
  (+ x y))  ; x and y bound from captured scope
; => 7

;; Useful for one-off evaluations without polluting namespace

ep-repl - Sub-REPL (JVM only)

(sc.repl/ep-repl 1)
; SC[1 -1]=> x
; => 3
; SC[1 -1]=> :repl/quit

Common Patterns

Spy without wrapping expression

(let [x (foo)
      y (bar x)]
  (sc.api/spy)  ; Just capture x and y here
  (something x y))

Spy only on errors

(try
  (my-fallible-code)
  (catch Throwable err
    (sc.api/spy err)  ; Capture scope when error occurs
    (throw err)))

Disable in loops

;; If spy/brk is in a loop, disable after first capture
(sc.api/disable! -1)  ; Use the Code Site ID (negative)

Combine multiple scopes

;; Capture from different places, then combine
(sc.api/defsc 1)  ; Defines a, b from first spy
(sc.api/defsc 2)  ; Defines x, y from second spy
;; Now all four vars available

Restore last execution point

(eval `(sc.api/defsc ~(sc.api/last-ep-id)))

Utility Functions

;; Get info about execution point
(sc.api/ep-info 1)
; => {:sc.ep/id 1
;     :sc.ep/local-bindings {x 3, y 4, ...}
;     :sc.ep/value 7
;     :sc.ep/code-site {...}}

;; Get info about code site
(sc.api/cs-info -1)

;; Get last execution point ID
(sc.api/last-ep-id)

;; Quiet versions (only log types, not values)
(sc.api/spyqt expr)
(sc.api/brkqt expr)

;; Clean up vars created by defsc
(sc.api/undefsc 1)

;; Free memory from execution point
(sc.api/dispose! 1)

Caveats

Memory leak warning: Remove spy/brk before production - they accumulate captured data.

Dynamic vars: Must be explicitly declared:

(sc.api/spy {:sc/dynamic-vars [*out* *my-var*]} expr)

defsc overwrites vars: If a var with same name exists (especially defonce), defsc will fail or overwrite it.

ClojureScript: Must use [ep-id cs-id] vector syntax:

;; ClojureScript requires both IDs
(sc.api/defsc [1 -1])
(sc.api/letsc [1 -1] expr)

Workflow Summary

1. Add spy where you want to capture: (sc.api/spy expr) or just (sc.api/spy)
2. Execute the code path (call the function, run the test, etc.)
3. Note the EP ID from the log: SPY [1 -1]
4. Recreate scope: (sc.api/defsc 1)
5. Debug by evaluating sub-expressions in your editor
6. Clean up: (sc.api/dispose! 1) and remove the spy call

Quick Reference