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I'm building a game in Purescript, using purescript-signal, which includes movement. The user presses the left/right key to move left/right. Minimal code is below.

It looks like purescript is evaluating the signal over "from the beginning of time" every step, which is baffling me. For example, if I keep pressing the right key at the beginning, the output is

m: 0
m: 0
m: 1
m: 0
m: 1
m: 2
m: 0
m: 1
m: 2
m: 3

rather than

m: 0
m: 1
m: 2
m: 3

as I would expect. How do I fix this?

module SimpleMove where

import Prelude
import Control.Monad.Eff (Eff)
import Control.Monad.Eff.Console (CONSOLE, log)
import Data.Functor
import Data.Int
import Signal (Signal, runSignal, foldp, sampleOn, map2)
import Signal.DOM (keyPressed)
import Signal.Time (Time, second, every)
import Partial.Unsafe (unsafePartial)

--MODEL
type Model = Int

step :: forall e. Partial => Int -> Eff (console :: CONSOLE | e) Model -> Eff (console :: CONSOLE| e) Model
step dir m' = 
  do
    m <- m'
    log ("m: " <> (show m))
    pure (m + dir)

--SIGNALS
inputDir :: Eff _ (Signal Int)
inputDir = 
    let 
        f = \l r -> if l 
                    then -1 
                    else if r
                         then 1
                         else 0
    in
      map2 f <$> (keyPressed 37) <*> (keyPressed 39)

input :: Eff _ (Signal Int)
input = sampleOn (every second) <$> inputDir

--MAIN
main :: Eff _ Unit
main =
    unsafePartial do
      dirSignal <- input
      let game = foldp step (pure 0) dirSignal
      runSignal (map void game)
holdenlee
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1 Answers1

3

If you change your main and step like this you'll get the expected result:

main :: Eff _ Unit
main = do
      dirSignal <- input
      let game = foldp step 0 dirSignal
      runSignal (map render game)

step :: forall e. Int -> Model -> Model
step dir m = m + dir

render :: forall e. Model -> Eff (console :: CONSOLE| e) Unit
render m = logShow m
soupi
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