#lang racket/base

;; A debug print syntax, including printing only when a condition or
;; counter is reached.
;;
;; The counter shows the use of `syntax-local-lift-expression` to make
;; a definition not at the invocation site, but up at the module
;; level.

(require (for-syntax racket/base
                     syntax/parse))

(define-syntax (dbg stx)
  (syntax-parse stx
    [(_ e:expr ...+)
     #'(begin
         (printf "`~a`=~v " 'e e) ...
         (newline))]

    [(_ #:when condition:expr e:expr ...+)
     #'(when condition
         (printf "[#:when `~a`] " 'condition)
         (dbg e ...))]

    [(_ #:count goal:nat e:expr ...+)
     #:with count (syntax-local-lift-expression #'0)
     #'(begin
         (when (= goal count)
           (printf "[#:count ~v] " count)
           (dbg e ...))
         (set! count (add1 count)))]))

;; Example use
(for ([s '("hi" "there" "blah" "foo" "bar")]
      [n (in-naturals)])
  (dbg #:when (= 2 n)
       n s)
  ;; Similar, handier when you don't already have some counter:
  (dbg #:count 2
       s))