Demystifying Edebug

Disclaimer

• My knowledge of Elisp (or any other Lisp) is not deep.
• Objective is to share knowledge and gain understanding.

Edebug

• Source-level debugger for Elisp
• On paper can do all the things you expect a debugger to do

Edebug though is a mess…

• Terrible ergonomics
• More a kit of parts than an actual tool
• Mismatches with contemporary naming

Edebug Fundamentals

• Edebug operates modally
• To debug, a function must be instrumented
  (C-u C-M-x)
• Any call to an instrumented function activates Edebug (Typically started by C-M-x)

Edebug Fundamentals (cont.)

• When Edebug is activated:
  • Different execution (run) modes available, among them:
    • Step
    • Trace
    • Go

Stepping (Jumping)

• Edebug stepping works with sexps
  • Forward one expression (f)
  • Step in to next expression after point (i)
  • Step out of containing expression (o)
• ❗️Different from debugging other languages

Watching (Evaluation List)

• Sets up a separate buffer of expressions to watch (E)
• ❗️Manual setup only, compared to IDEs - more bindings
• Persisted for Emacs session

Breakpoints

• Must instrument function before setting a breakpoint
• ❗️Breakpoints are only visible when Edebug is activated
• ❗️Re-evaluating a function removes all previous breakpoints

Misc Commands

• Show stack trace (backtrace) (d)
• Repeat last result (r)
• Prompt for expression (e)

Edebug User Experience

• Steep learning curve
• Many keybindings and commands to know ahead of time
• Difficult to discover and recall

It’s 2024, we can do better

• Add Transient menus for Edebug
• Prototype

Demo