Mirror of forum.nim-lang.org

7017 :: Help using the effects pragma

• fxn (orginal) [2020-10-31T08:26:10+01:00] view original

  I am trying to understand why does this proc may have side-effects:

  import times
  
  const gigasecond = 1_000_000_000.seconds
  
  proc addGigasecond*(moment: DateTime): DateTime =
      moment + gigasecond
  Run

  If you change proc with func you get:

  
  Error: 'addGigasecond' can have side effects
  Run

  I was told that the problem is in +, but from the point of view of the user I do not quite understand where's the side-effect. If you pass a particular DateTime object, in what sense does adding a fix number of seconds may have a side-effect? Isn't the result deterministic?

  I tried using the effects pragma like this:

  import times
  
  const gigasecond = 1_000_000_000.seconds
  
  proc addGigasecond*(moment: DateTime): DateTime =
      result = moment + gigasecond
      {.effects.}
  Run

  but I see nothing in the output when compiling the file. Am I using the pragma correctly?

---

• juancarlospaco (orginal) [2020-10-31T14:29:43+01:00] view original

  You may not always escape having side effects, that happens even on Functional-only langs.

---

• Araq (orginal) [2020-10-31T14:45:26+01:00] view original

  The .effects pragma only lists exceptions and "tag" effects.

  import times
  
  const gigasecond = 1_000_000_000.seconds
  
  proc g() {.tags: [RootEffect].} = discard
  
  proc addGigasecond*(moment: DateTime): DateTime =
      result = moment + gigasecond
      g()
      {.effects.}
  Run

  (I'm not sure why either but that's how it is.)

---

• fxn (orginal) [2020-10-31T15:44:21+01:00] view original

  That's a pretty generic statement. Can you be more precise in this particular case?

---