Tools to develop and debug ELISP code

Question

I have some experience in Emacs-LISP. Today I spent 6 hours writing LISP code that I guess-timate would have taken 1h30 to write in Python because my workflow to develop and debug is very clunky. In Python I would log successive values with print("Stage N: " + value) at different stages. I am looking for something better.

I give two examples below. I'm sure other people are more productive given the high reputation of LISP. What workflows do coders use for development and debugging in Emacs-LISP?

One example

Automated testing with ERT does not indicate the line of an error. This is so strange that I made it into its own question).

Another example

In debugging a function with an optional argument, I was unable to log that value to the *Messages* buffer as I could not find how to convert t and nil to string. In the *scratch* buffer:

(message (concat "Value is: " (string t))) ;; concat: Wrong type argument: characterp, t

(message (concat "Value is: " (string nil))) ;; concat: Wrong type argument: characterp, nil

(message (concat "Value is: " t)) ;; message: Wrong type argument: sequencep, t

(message (concat "Value is: " nil))
"Value is: "

Answer 1

Here is the beginnings of a list of useful techniques culled from the comments. Please add any others that you find useful by editing this answer:

• The elisp analog of sprinkling printfs in a program to show values of variables: (message "Value is %s" whatever).

• Using a debugger: possibilities include the Lisp Debugger, and edebug.