Log Once, a debugging utility for Racket

log-once is a macro that allows the user to log both an expression and its value to the console, as well as to specify conditions under which to log. The macro is most effective in tight loops, where a naive logger would saturate stdout.

Furthermore, the macro can be configured to stop logging after a given number of invocations, using a lifted boolean to track the log count even after leaving the expression's scope.

Type signature

Argument	Type	Required?	Default value	Description
#:skip-count	Natural	False	0	Assuming other logging conditions are met, the number of invocations to ignore before starting to log
#:log-count	Natural	False	1	Assuming logging conditions are met, the number of times that calling log-once will log the expression
#:when	Condition	False	#t	log-once will try to log the expression if this condition is met
#:message	String	False	""	A string literal to write to stdout before writing the expression
#:newline/def	Boolean	False	False	Whether to write a newline between every logged expression
#:newline/defs	Boolean	False	True	Whether to write a trailing newline after writing all expressions
#:return	Expression	False	Void	The value to return from the invocation of log-once. Intended to be used for inline invocations
expr ...	Expression	True	-	The expressions whose syntax and values should be logged to the console

Examples

Waits until n is greater than 10, then log the following two values

(require log-once)

(for ([n (in-range 20)])
  (log-once n
            #:when (> n 10)
            #:log-count 2))

#| IN STDOUT:
n = 11
n = 12
|#

Use log-once in a nested scope without losing track of the log count

(require log-once)

(define (function-scope str)
  (log-once (~a #:align 'center #:width 20 str)))

(function-scope "first string")
(function-scope "second string")

#| IN STDOUT:
(~a #:align (quote center) #:width 20 str) = "    first string    ". 
|#