map over values within signals

[obcan]

Great work! Is possible map over produced values within signal like in RxJS functors? I mean something like this:

import Prelude hiding (div)
import Signal
import Signal.DOM

doubleCoordinates c = c {x = c.x * 2, y = c.y * 2}

-- | produce new signal with doubled coordinates from origin
main = mousePos ~> doubleCoordinates

[michaelficarra]

Yes, exactly what you've written should work. What's wrong?

[obcan]

This pseudocode produce type mismatch Error. Function passed to functor ~> must be type of Signal CoordinatePair -> Signal CoordinatePair but doubleCoordinates have type CoordinatePair on the right side. Maybe I missed something and only one way is use to Foldp.

[kritzcreek]

Your code fails because of the type of mousePos:

mousePos :: forall e. Eff (dom :: DOM | e) (Signal CoordinatePair)

As you can see the Signal is "wrapped" inside an Eff which also has a functor instance. The ~> or map now tries to apply doubleCoordinates to a Signal CoordinatePair instead of a CoordinatePair. We can solve this by first extracting the mouse Signal from mousePosition and then mapping.

import Prelude hiding (div)
import Signal
import Signal.DOM

doubleCoordinates c = c {x = c.x * 2, y = c.y * 2}

-- | produce new signal with doubled coordinates from origin
main = do
  mouse <- mousePos
  pure (mouse ~> doubleCoordinates)