The Clojure debug-repl

Every time I stick a println into some Clojure code to debug it, I think to myself, "This is Lisp! I should be able to insert a repl here!"

The problem is of course that Clojure's eval function doesn't know about the surrounding lexical scope. How to solve the problem? Create a macro that passes a copy of the lexical scope in with the form to be evaled, something like this:

(defn eval-with-locals [locals form] (eval `(let ~(generate-local-bindings locals) ~form)))

USE: The interface is meant to be dead simple: "(use 'alex-and-georges.debug-repl)" loads it, and "(debug-repl)" invokes it.

That's about it. When you enter the debug-repl, the regular repl prompt will be replaced with

dr =>

An example will make it clearer:

user=> (let [c 1 d 2] (defn a [b c] (debug-repl) d)) #'user/a

user=> (a "foo" "bar")

dr-1-1001 => c "bar"

dr-1-1001 => d 2

dr-1-1001 => locals {fn__104 #<user$eval__103 user$eval__103@5f6303>, c "bar", d 2, fn__106 #<user$eval__103$a__105 user$eval__103$a__105@179dce4>, b "foo", counter__56__auto__ 1001}

dr-1-1001 => (str b c) "foobar"

dr-1-1001 => () 2 user=>

LIMITATIONS: The debug-repl doesn't currently integrate properly with the slime-repl, (I think because of how Slime manages IO redirection,) so you'll have to invoke it from a regular repl, or slime's inferior lisp buffer.

A version of the debug-repl has been ported to slime. More details here: http://hugoduncan.org/post/2010/swank_clojure_gets_a_break_with_the_local_environment.xhtml