How does `valueB` work? It always returns the same value?

Question

Consider the following code:

module Main where

import Control.Event.Handler
import Reactive.Banana
import Reactive.Banana.Frameworks

main :: IO ()
main = do
  (addHandler, fire) <- newAddHandler
  compile (network addHandler) >>= actuate
  fire "fire one"
  fire "fire two"

network :: AddHandler String -> MomentIO ()
network addHandler = do
  fireEvent <- fromAddHandler addHandler
  behavior <- stepper "initial stepper value" fireEvent
  behaviorValue <- valueB behavior
  reactimate $ putStrLn . (\v -> "fire input: " ++ show v) <$> fireEvent
  reactimate $ print behaviorValue <$ fireEvent

The output from this program is:

fire input: "fire one"
"initial stepper value"
fire input: "fire two"
"initial stepper value"

Am I using valueB correctly? If not, what am I doing wrong? What is the purpose of valueB and when should I use it?

Answer 1

It helps to keep in mind that the Moment (and MomentIO) monad expresses a computation that happens at a particular moment in time. The combinator valueB simply returns the value of the Behavior at that moment. In your case, that is at the very beginning.

If you want to sample the value of a Behavior at different times, you can use the <@> and <@ operators, which are very similar to <$> and <$. For instance, replacing the last line with

reactimate $ print <$> behavior <@ fireEvent

will print the value of the Behavior at each moment when the event fires.

A more advanced use would be to use valueB with execute.