Elm's Time is not Precise, is it? - time

Question

I've been toying with Effects.tick and Time in one of my apps, and I can't seem to get my delays to be around the Time.second that it claims to be. It's nearly immediate. I understand that type alias Time = Float, and from my logs it seems that second = 1000, but this just burns so quickly, even with logging. Is there a clear explanation for this?

Answer 1

Effects.tick causes an action to be called nearly instantaneously, but the action that is being called gets passed a value of the current time of the tick. If you want to delay something by a second while using Effects.tick, you'll have to keep track of a starting point and compare it to the time of the current tick, and that's where you can add in Time.second.
Take this arbitrary example (you can paste it into http://elm-lang.org/try):
import Html exposing (..)
import Html.Events exposing (..)
import Html.Attributes exposing (..)
import StartApp
import Effects exposing (Never)
import Task
import Signal
import Time exposing (..)
app =
StartApp.start
{ init = init
, view = view
, update = update
, inputs = [ ]
}
main =
app.html
type alias Model =
{ lastTick : Maybe Time
, tickEverySecond : Maybe Time
}
init =
({ lastTick = Nothing, tickEverySecond = Nothing }, Effects.tick MyTicker)
view address model =
div []
[ div [] [ text <| "Current tick: " ++ (toString model.lastTick) ]
, div [] [ text <| "Updated every second: " ++ (toString model.tickEverySecond) ]
]
type Action
= MyTicker Time
update action model =
let
everySecond = Maybe.withDefault 0 model.tickEverySecond
getTickEverySecond time =
if time > everySecond + Time.second then
Just time
else
Just everySecond
in
case action of
MyTicker time -> (
{ model | lastTick = Just time
, tickEverySecond = getTickEverySecond time
}, Effects.tick MyTicker)
port tasks : Signal (Task.Task Never ())
port tasks =
app.tasks
Every update call requests a new tick, so it will just spin forever. The important part is that tickEverySecond is only updated if the last time it was updated is greater than one second ago.
You ask about the time precision. If you run that example, you'll notice that the increment every second is somewhat approximate; it will drift greater than a second. That isn't due to any kind of underlying imprecision. Remember that Elm's Time functionality is a thin veneer over Javascript's time and timer functionality. That drift is merely an artifact of all the tiny little delays caused by running code that responds to a timer signal.