Request to add instrumentation option for local variables about postmortem HOT 2 CLOSED

Hi @Futurile, Thank you for giving a try to the library! And thanks for your feedback, too. 😉

I think it's hardly possible for instrument to programmatically determine where the call to dump should be inserted into because a function may have several different local contexts and they can even be nested. For instance, every call to dump occurring in the example code below will all save different log data:

(defn f [x]
  (pm/dump :f) ;; saves {:x 0} when you call f with 0
  (let [x 10]
    (pm/dump :f) ;; saves {:x 10}
    (let [x 20]
      (pm/dump :f))) ;; saves {:x 20}
  (let [x 30]
    (pm/dump :f))) ;; saves {:x 30}

And moreover, instrument doesn't even have access to the code of a function, so it can't insert any code into the middle of the function code. After all, I would say it has to be considered as the user's responsibility to determine where the calls to dump should be inserted into.

As an alternative solution (though I think it's not a good one), how about naming the log entry key same as the name of the function that you want to instrument? If you do so, you can collect logs both for dump and instrument into a single log entry:

(defn f [x y]
  (let [x2 (* x x) y2 (* y y)]
    (pm/dump `f)
    (Math/sqrt (+ x2 y2))))

(pi/instrument `f {:xform (map (fn [m] (if (:args m) m {:locals m})))})
(f 3 4) ;=> 5
(pm/log-for `f)
;=> [{:args (3 4)} {:locals {:x 3, :y 4, :x2 9, :y2 16}} {:args (3 4), :ret 5.0}]

from postmortem.

Thanks for the explanation @athos

I am probably unclear (or poorly understand) how instrument works. I thought it "somehow" changed execution of the function, so that it was placing a spy>> right at the start of the function and also right at the end.

"the execution log for an instrumented function consists of two types of log items, ones for entry and ones for exit"

Consequently, I imagined that what I was asking was that instrument would enable the dump right at the start and end of the function. So from your example:

(defn f [x]
  (pm/dump :f) ;; saves {:x 0} when you call f with 0
  (let [x 10]
    (let [x 20]
  (let [x 30]
    (pm/dump :f))) ;; saves {:x 30}

I see what you are saying - I wouldn't see the specific values being used if there's another let later on as your example showed. In particular, any let's at the start of the function wouldn't be shown which I guess is pretty common.

I am using it as a learning tool, to play with simple function, so hadn't really thought about more complex use-cases.

from postmortem.