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5020 :: side effects through local() or parse() from times module

• dschaadt (orginal) [2019-07-16T17:01:10+02:00] view original

  Hi, I'd like to write a function that extracts the date from a DateTime object returns a new DateTime object with that date but time set to 00:00:00. The input will be in unixtime due to other requirements. When I use the following

  func test(unixTime: int64): bool =
    let t1 = local(fromUnix(unixTime))
  #    let t = initDateTime(t1.monthday, t1.month, t1.year, 0, 0, 0).toTime.toUnix
  Run

  local() leads to side effects. Using instead

  parse(format(fromUnix(unixTime), "yyyy-MM-dd"), "yyyy-MM-dd").toTime.toUnix
  Run

  also leads to side effects and is slower anyways.

  Is there a way to circumvent these side effects such that I can use a func instead of proc? Thanks!

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• treeform (orginal) [2019-07-16T18:51:00+02:00] view original

  I think it leads to side effects because it uses clib's time functions. I have a pure nim implementation of times, calendars and timezones here: https://github.com/treeform/chrono

  import chrono
  
  proc test1(unixTime: int64): bool =
    var
      t = Timestamp(unixTime) # in seconds?
      c = t.calendar
    c.toStartOf(Day)
    echo c.ts
    echo c.formatIso
  
  func test(unixTime: int64): bool =
    var c = Timestamp(unixTime).calendar # convert to calendar
    c.toStartOf(Day) # rewind calendar to start of day
    c.ts.float == 0 # convert calendar to timestmap, then to float
  
  discard test1(1233456789)
  discard test(1233456789)
  Run

  But honestly it might be easier to just quantize your thing by a day if you don't care about timezones and daylight savings.

  unixTime - unixTime mod (24*60*60)
  Run

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• Araq (orginal) [2019-07-16T18:58:15+02:00] view original

  To circumvent the effect system, you can use this (since 0.20.0):

  func test(unixTime: int64): bool =
    {.noSideEffect.}:
      let t1 = local(fromUnix(unixTime))
  
  Run

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• dschaadt (orginal) [2019-07-16T22:17:54+02:00] view original

  Thanks for the quick responses. When I try to use chrono, it complains that miniz is missing, so it that truely pure nim? Could it work without miniz. Unfortunately, I cannot use an approximate solution without timezones since the code will be used to calculate the moon rise and set time for a given position with it's local timezone... So I'll probably stick to the solution by Araq

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• GULPF (orginal) [2019-07-16T22:42:34+02:00] view original

  This happens because the proc fields of times.Timezone isn't marked with noSideEffect. The same thing happens if you try to do anything with streams inside a func, since streams also use proc fields without noSideEffect. This could be changed, but imo Nims effect system isn't worth bothering with, just use proc instead. Every time I try to use the effect system I just get frustrated.

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• juancarlospaco (orginal) [2019-07-16T23:11:13+02:00] view original

  If you know how to improve it, then send the improvements. If its not perfect does not meant is useless. IMHO.

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• treeform (orginal) [2019-07-17T04:29:41+02:00] view original

  "moon rise and set time" oh I have another library for you:

  https://github.com/treeform/orbits

  You can see adapt JPL Horizon api to get the exact "moon rise and set time" times directly from Nasa.

  You right about chrono and miniz. Miniz is just there to make timezone data smaller. I should make it optional.

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• dschaadt (orginal) [2019-07-19T15:36:15+02:00] view original

  For those that are interested, I was finally able to implement the routines to calculate moonrise and moonset along with other astronomical data. You can find my small module here https://github.com/dschaadt/astro

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