How to propagate clicks signal to model update in Elm? - elm

Question

I'm writing a game in Elm and in this game there's a button that when pressed should reset the game board to its initial state. I don't understand however how a button click signal is to be propagated to the board model's update function. In the code below, I've just passed the unit value to stepBoard but I can't do much with it so how can I solve it?
--MODEL
type Board = [(Int,Int)]
type Game = {name: String, board: Board}
defaultGame = {name = "Test", board = [(3,3)]}
--UPDATE
stepBoard click (colsInit, rowsInit) (rows, cols) g =
{g | board <- if ? then --some initial value (colsInit, rowsInit)
else --some updated value using (rows, cols)}
stepGame: Input -> Game -> Game
stepGame {click, name, rowsColsInit, rowsCols} g = stepBoard click (0,0) (0,0) g
game = foldp stepGame defaultGame input
--INPUT
type Input = {click:(), name: String, rowsColsInit: (Int,Int), rowsCols: (Int,Int)}
input = Input <~ clicks ~ nameSignal ~ rowsColsInitSignal ~ rowsColsSignal
--VIEW
(btn, clicks) = Input.button "Click"

Answer 1

Combining foldp and click information can be unintuitive.
The usual solution employed is an ADT to encode the different possible events:
data Input = Init (Int, Int)
| Update { name : String
, rowsCols : ( Int, Int )
}
input = merge (Init <~ (sampleOn clicks rowsColsInitSignal))
(OtherInput <~ nameSignal ~ rowsColsSignal)
merge merges two signals and always creates a constant function.
Now you can just match by case in your update function:
stepBoard input g =
let newBoard =
case input of
Init (initRows, initCols) -> -- do your initialisation here
Update {name, rowsCols} -> -- do your update here
in { g | board <- newBoard }