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. So I started asking myself, what
is the simplest change I could make to Clojure to support an eval that
understands that scope? Then I tried to implement it.
Basically, here's what I came up with.
1. Modify the Clojure compiler so that when a flag is turned on, it
stores references to the lexical scope in a dynamic var. Thus, each
time the compiler creates a new lexical scope, it also emits the byte
code to push a hash-map with the details onto the var. When that scope
ends, the byte code for popping the hash-map off the var is emitted.
2. Then in Clojure proper, add a special version of eval that uses
that var. It wraps the form being eval'ed in a "let" that emulates the
original lexical scope, something like this:
`(eval
(let [~@(make-let-bindings
(:lexical-frames (var-get (resolve context))))]
~form))
With those two pieces, it's straight-forward creating a "debug-repl"
that understands the surrounding lexical scope.
I'm pretty pleased with the results and wanted to show them off. More
details here:
http://georgejahad.com/clojure/debug-repl.html
Thanks to the my coworkers, the "Sonian-Clojure Brain Trust", for
their support and encouragement!
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