How to loop through dates in Racket? - date

Question

I want to loop through dates from the current date to some stop date, like the following example in Python:
import datetime
def count_dates(stop_date):
step = datetime.date.today()
while step >= stop_date:
yield step
step = step - datetime.timedelta(days=1)
def main():
for step in count_dates(datetime.date(2018, 1, 1)):
print(step.isoformat())
I don't see any date calculation functions in Racket, however. How can this be done?

Answer 1

If you want to use racket/date, you can accomplish your goal with find-seconds, seconds->date, current-seconds, and a simple arithmetic:
#lang racket
(require racket/date
racket/generator)
(define 1day (* 60 60 24))
(define (count-dates year month day)
(define target (find-seconds 0 0 0 day month year))
(in-generator
(let loop ([current (current-seconds)])
(when (>= current target)
(yield (seconds->date current))
(loop (- current 1day))))))
(date-display-format 'iso-8601)
(for ([dt (count-dates 2019 2 1)])
(displayln (date->string dt)))
This outputs:
2019-02-11
2019-02-10
2019-02-09
2019-02-08
2019-02-07
2019-02-06
2019-02-05
2019-02-04
2019-02-03
2019-02-02
2019-02-01
That is, from today (Feb 11, 2019) to Feb 1, 2019.
You can also use gregor or srfi-19 which are alternative date libraries that have a notion of time duration/difference.

Answer 2

Here is how you can do it using the date arithmetic provided by gregor. Specifically the +days and -days functions are helpful here:
#lang racket
(require gregor
racket/generator)
;; Date -> [Sequenceof Date]
(define (count-dates stop-date)
(in-generator
(let loop ([step (today)])
(when (date>=? step stop-date)
(yield step)
(loop (-days step 1))))))
(for ([step (count-dates (date 2018 1 1))])
(displayln (date->iso8601 step)))
And if you need something more general than "some number of days", there's also functions like +date-period and -date-period.