Parameters in Racket for debug-printing in recursive functions

racket-parameters-debug-print-indentation.rkt

#lang racket

#|

I'm trying to understand parameters in Racket.

One idea: for a recursive function,
have a parameter that says how much to indent a debugging print
statement so that I can see the call stack more clearly.

https://beautifulracket.com/explainer/parameters.html

https://www.greghendershott.com/2013/04/parameters-in-racket.html

|#

(define current-indent-level (make-parameter 0))
(define current-print-debug? (make-parameter #f))

(define (indentation n)
  (string-join (build-list n (λ (_) "  ")) ""))

(define (debug-printf . args)
  (when (current-print-debug?)
      (printf (string-join (list (indentation (current-indent-level))
                             (apply format args))))))

(define (fib n)
  (debug-printf "fib called ~s\n" n)
  (cond [(or (equal? n 0) (equal? n 1))
         n]
        [else
         (begin
           (parameterize ([current-indent-level (add1 (current-indent-level))])
             (+ (fib (- n 1)) (fib (- n 2)))))]))

#|

Nicest way to change a parameter for some scope is parameterize:

    (parameterize ([current-print-debug? #t])
      (fib 4))

Or just change it globally:

(current-print-debug? #t)

...which is roughly the equivalent of this thing I do in Python:

    DEBUG=True

    ...

    if DEBUG: print("blah")

and I toggle the global variable.

|#

(current-print-debug? #t)

(fib 4)