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06__Time_Signals.md

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Time Signals

The Time module from the standard library defines functions and signals related to working with time. Elm defines the Time type as an alias for Float. A value of that type denotes a given number of milliseconds. However, we do not have to remember that the time values are expressed in milliseconds. The Time modules provides functions returning units of time: millisecond, second, minute, hour. You can multiply their results by a given number to get the requested duration:

  > import Time (..)
  > millisecond
  1 : Float
  > 2*second
  2000 : Float
  > 4*minute
  240000 : Float
  > hour
  3600000 : Float

There are also functions which convert values in the opposite direction:

  > inSeconds 2000
  2 : Float
  > inMinutes 90000
  1.5 : Float
  > inHours 7200000
  2 : Float
  > inMilliseconds 4
  4 : Float

Besides the above functions, the Time module defines functions which create time related signals. The TimeSignals.elm program (a working example is available here), presents a few examples of their use.

% TimeSignals.elm module TimeSignals where

  import Graphics.Element (down, flow)
  import List (map)
  import Mouse
  import Signal ((~), (<~))
  import Text (plainText)
  import Time (delay, every, fps, fpsWhen, second, since, timestamp)


  showsignals a b c d e f =
      flow
          down
          <|
              map
                  plainText
                  [
                      "every (5*second): " ++ toString a,
                      "since (2*second) Mouse.clicks: " ++ toString b,
                      "timestamp Mouse.isDown: " ++ toString c,
                      "delay second Mouse.position: " ++ toString d,
                      "fps 200: " ++ toString e,
                      "fpsWhen 200 Mouse.isDown: " ++ toString f
                  ]


  main = showsignals
             <~ every (5*second)
             ~ since (2*second) Mouse.clicks
             ~ timestamp Mouse.isDown
             ~ delay second Mouse.position
             ~ fps 200
             ~ fpsWhen 200 Mouse.isDown

The first signal, every second, outputs the current timestamp every 5 seconds.

The second signal is a boolean signal, that is True for 2 seconds after every mouse click. The since function takes a Time value t and another signal and outputs a Boolean signal which is true during time t after every event from the input signal.

The third signal outputs pairs of values: the first element of the pair is the current timestamp, and the second one is the value of the Mouse.isDown signal.

The fourth signal outputs Mouse.position delayed by one second. The delay function takes a Time value and a signal, and creates another signal which outputs the same events as the input signal, but delayed by the given time.

The fifth signal outputs events as fast as possible, but no more than 200 per second (on fast computers it will be approximately 200 events per second, but on slower computers it may be less than that). The value emmitted as each event, is the time difference the time the current event is emitted and the time the previous event was emitted.

The last signal is like the previous one, but it only emits events when the mouse button is pressed.

The next chapter presents an example program that is using one of the functions presented above.