trace facility in Racket?

[dgtlcmo]

Does anyone know how to trace functions inside a closure?  I would like to trace hanoi-move, however find that with MIT Scheme I need to place (trace) within the closure (hanoi n), otherwise the trace will not occur.

Can a trace like this be performed in Racket?

Thanks

 (define (hanoi n)

  (define (pmd from to)

    (display "Move ")

    (display from)

    (display " to ")

    (display to)

    (newline)

    '())

  (define (hanoi-move n from to spare)

    (cond ((= n 0) '())

 ((= n 1) (pmd from to))

 (else

  (hanoi-move (- n 1) from spare to)

  (hanoi-move 1 from to spare)

  (hanoi-move (- n 1) spare to from))))

  (hanoi-move n "A" "B" "C"))

[Ben Greenman]

Yes, Racket has trace tools:
https://docs.racket-lang.org/reference/debugging.html

Try adding (trace hanoi) before the call to (hanoi-move ....) at the bottom

[Nicholas Papadonis]

I was successful with MIT Scheme placing (trace proc) inside the closure of this code.  Placing (trace proc) outside the closure did not have any effect.

This is the question I'm asking.  Is it possible to trace without modifying the closure?

I ask due to forward thinking about software engineering practices and modifying code (although it could be argued in C a preprocessor block could be placed to compile in or out conditional macros blocks).

Thanks again

[John Clements]

It sounds like you’re looking for a way to have the trace inserted for you by a debugging tool, so you don’t have to remember to take it out again later. This wouldn’t be hard to do, but (as far as I know) no one’s taken the time to do it.

John

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[Nicholas Papadonis]

Thanks, you answered my question. I’m educating myself in ways to debug Scheme, coming from an imperative background.

I suspect the solution would be to leave the trace statements in and use a conditional when delivering production code.

[John Clements]

Right, sorry, I should have added… doing *that* (eliminating the traces for production code) is almost trivial: just make a macro that’s defined to disappear when a flag is set:

#lang racket

(define-for-syntax production-flag #f)

(define-syntax (debug-only stx)
(syntax-case stx ()
[(_ expr)
(if production-flag
#'(void)
#'expr)]))


(define (fact f)
(if (= f 0)
1
(begin
(debug-only (printf "hello!\n"))
(* f (fact (- f 1))))))

(fact 6)

The hard part is building UI tools to manipulate and insert these things without any changes to the code at all.

John