榆树 - 信号的历史价值

Question

鉴于信号，人们如何获得其历史价值？

像

这样的东西

--current value
Signal.pastValue(0, Mouse.x)

--previous value
Signal.pastValue(1, Mouse.x)

--previous nth value
Signal.pastValue(n, Mouse.x)

我已尝试使用Signal.foldp，但它似乎会返回取决于事件编号的当前值或累计值。

Answer 1

Elm doesn't keep track of historical values on its own, but you could use foldp to build a list of any type of signal like this:

history : Signal a -> Signal (List a)
history =
  Signal.foldp (::) []

The most recent signal value is prepended to that list. To see it in action, you could past this full example into http://elm-lang.org/try

import Graphics.Element exposing (show)
import Mouse

main =
  Signal.map show <| history Mouse.x

history : Signal a -> Signal (List a)
history =
  Signal.foldp (::) []

Running that example may shed light on why historical values aren't kept by default: You can quickly bloat your memory. That being said, elm-reactors time-travelling debugger keeps history around, but only for debugging purposes. This isn't something you'd normally want in production.