Confusion with running tasks outside dom event handlers

[Brendan Zabarauskas]

I am trying to write a slideshow presentation that updates the URL hash based on the current slide number. I am trying to use the elm-history library to do it.

Here is the type signature of the function I would like to use:

setPath : String -> Task error ()

Unfortunately I have no idea how to call that on a key press. All the examples I have seen regarding Tasks use dom event handlers, but I don't want to use them.

Here is a reduced example of the thing I'm trying to do:

import History
import Graphics.Element exposing (Element, show)
import Mouse
import Signal exposing ((<~))
 
type alias Model = Int
 
init : Model
init = 0
 
update : (Int, Int) -> Model -> Model
update (x, _) model =
  let model' = x
      _ = History.setPath ("#" ++ toString x) -- Should alter the URL :/
  in
    model'
 
input : Signal (Int, Int)
input = Mouse.position
 
view : Model -> Element
view = show
 
main : Signal Element
main =
  view <~ Signal.foldp update init input

gist

I've attempted to use mailboxes, but didn't get very far.

[Corey Trampe]

You are creating a Task, and then just throwing it away. Try throwing it at a port instead.

http://elm-lang.org/guide/reactivity

[Brendan Zabarauskas]

Thanks! I rethought it, and this is what I came up with:

import History
import Graphics.Element exposing (Element, show)
import Mouse
import Signal exposing ((<~))

import Task exposing (Task)
 
makeTitle : Int -> String
makeTitle n = "n = " ++ toString n
 
port title : Signal String
port title = makeTitle <~ counts
 
makeHash : Int -> String
makeHash n = "#" ++ toString n
 
port runTask : Signal (Task error ())
port runTask = History.setPath <~ (makeHash <~ counts)
 
update : () -> Int -> Int
update () n = n + 1
 
counts : Signal Int
counts = Signal.foldp update 0 Mouse.clicks
 
main : Signal Element
main = show <~ counts

gist

I think the key was figuring out that I could flow the model signal through both main *and* runTask.

I might submit a PR to elm-history adding it as an example :)