Clojure: capture runtime value of function arg, to use in REPL

Question

Problem

My web front-end calls back-end queries with complex arguments. During development, to avoid time-consuming manual replication of those arguments, I want to capture their values in Vars, which can be used in REPL.

Research

This article shows that inline def is a suitable solution, but I couldn't make it work. After a call from the front-end happened, the Var remained unbound.

I launched the backend with REPL through VS Code + Calva, with the following code:

(defn get-analytics-by-category [params]
  (def params params)
  ...)

And here's the evaluation of the Var in the REPL:

#object[clojure.lang.Var$Unbound 0x1298f89e "Unbound: #'urbest.db.queries/params"]

Question

Why the code above didn't bound value of the argument to the Var? Is there another solution?

Try adding a `println` with the value of the arg to verify the function is being invoked, and you are getting the expected value from the frontend. — Alan Thompson, Jan 07 '21 at 16:56
@AlanThompson, already tried `println`, the function is invoked as expected. — Herman Nurlygayanov, Jan 07 '21 at 17:07
What about defining a custom Java class with public static field where you can put the value to read it later? — Rulle, Jan 07 '21 at 17:16

Herman Nurlygayanov · Accepted Answer · 2021-01-08T15:32:55.520

The best way that I found is to use scope-capture library. It captures all the local variables with addition of 1 line in a function, and then with another 1-liner you can define all those variables as global, which allows you to evaluate in REPL any sub-expression in the function using runtime values.

If you ever spent a lot of time reproducing complex runtime values, I strongly recommend watching their 8-min demo.

My issue with inline-def was likely caused by reloading namespace after the Var was bound to a value. After restarting VS Code and carefully doing everything again, the issue went away.

You may want to have a look at 'defonce', maybe. – Rulle Jan 07 '21 at 18:22 — Rulle, Jan 07 '21 at 18:22

score 0 · Answer 2 · answered Jul 14 '22 at 10:50

0

Another way to look at the runtime is to use a debugger.

It is more flexible than scope-capture, but requires a bit more work to make variables available outside an execution.

VS Code's Calva extension comes with one, there's also Emacs packages.

answered Jul 14 '22 at 10:50

Herman Nurlygayanov

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Clojure: capture runtime value of function arg, to use in REPL

Problem

Research

Question

2 Answers2