Elm Signals and Suggestions

Working online  In general, I found working online at to be the best way to program Elmhttp://elm-lang.org/try  Instant feedback, so you can try one change at a time and see if it worked  No good way to save your work; I did an occasional copy-all and paste into Sublime Text  The images used in the examples are on the server, not locally, so you can’t readily use them in your own program  I installed Elm and used the elm repl to test single expressions and very small bits of code  The REPL is especially useful for entering expressions to find out their type  > Signal.map : (a -> b) -> Signal.Signal a -> Signal.Signal b 2

Program structure  Elm programs have a structure very similar to MVC (Model-View-Controller)  The structure is Model - Update - View, or sometimes Model - Update - View - Signals  In Elm  The Model is typically just data, describing the current state of the program; typically this is in the form of a record named model  The Update is a set of pure functions; typically there is a function named update with a signature similar to Input -> Model -> Model  The View is a set of functions whose purpose is to display the current state; typically there is a function named view with a signature something like Input -> Model -> Element  The Signals part includes a function named main ; it collects signals together and bundles them up, sends them to update to get a new program state, and sends the result to view 3

Mouse signals  import Mouse exposing (..)  position : Signal ( Int, Int )  x : Signal Int  y : Signal Int  isDown : Signal Bool  clicks : Signal () 4

Keyboard signals  import Keyboard exposing (..)  Mouse.position : Signal (Int, Int)  arrows : Signal { x : Int, y : Int }  wasd : Signal { x : Int, y : Int }  enter, space, ctrl, shift, alt, meta are all Signal Bool  type alias KeyCode = Int  isDown : KeyCode -> Signal Bool  keysDown : Signal (Set KeyCode)  presses : Signal KeyCode -- most recent key pressed 5

Time signals  import Time exposing (..)  fps : number -> Signal Time  fps (frames per second) will produce a signal the given number of times every second  fpsWhen : number -> Signal Bool -> Signal Time  Same as the fps function, but you can turn it on and off  every : Time -> Signal Time  Takes a time interval t and produces a signal updated every t  delay : Time -> Signal a -> Signal a  Delays a time signal  There a few additional functions 6

Window signals  import Window exposing (..)  dimensions : Signal ( Int, Int )  width : Signal Int  height : Signal Int  When you embed Elm in a it gives the dimensions of the container, not the whole window 7

Monads  Signal is a monad type  Signals, by definition, vary over time  Clearly, this is inappropriate for a pure functional language  Monads “isolate” impure operations  The main function in the Signals part of an Elm program: 1. Collects signals together and bundles them up 2.Extracts the data from the signals and sends them to the pure function update to get a new program state 3.Sends the new program state to view 8

Bind  Remember “bind” ( >>= ) in Haskell?  bind takes a value out of a monad, applies a function to it, and puts the result back into a monad: (>>=) :: Monad m => m a -> (a -> m b) -> m b  Elm has something similar but not identical  Signal.map : (a -> b) -> Signal a -> Signal b  This allows us to take a Signal a, use it to call a pure function (a -> b), and get the result as a Signal b  The arguments are reversed from those of >>= but in the same order as those of List.map  List.map : (a -> b) -> List a -> List b  <~ is an alias for Signal.map 9

Signal.map n  Signal.map applies a pure function to the value inside a Signal, producing another Signal  Signal.map : (a -> b) -> Signal a -> Signal b  However, you often have pure functions that take more than a single argument  map2 : (a -> b -> result) -> Signal a -> Signal b -> Signal result  The result type is, of course, up to the supplied function, but is often a tuple  map3 : (a -> b -> c -> result) -> Signal a -> Signal b -> Signal c -> Signal result  map4 : (a -> b -> c -> d -> result) -> Signal a -> Signal b -> Signal c -> Signal d -> Signal result  map5 -- the obvious type is omitted for reasons of space 10

Warning! lift  Elm is a new and rapidly changing language  If you find examples using the functions lift, lift2, lift3, etc., these functions no longer exist  They have been replaced by map, map2, map3, etc.  The old functions have the same signature as the new, and as far as I know, is just a name change 11

Sampling  import Signal exposing (..)  sampleOn : Signal a -> Signal b -> Signal b  Sample from the second input every time an event occurs on the first input. For example, (sampleOn clicks (every second)) will give the approximate time of the latest click  The value of the first signal ( Signal a ) is discarded  I believe that the purpose of sampleOn is to restrict the number of events that must be handled by the update and view parts of the program  Mouse.position can probably produce hundreds of events a second 12

foldp  foldp folds signals “over time”  From the documentation: foldp : (a -> state -> state) -> state -> Signal a -> Signal state Create a past-dependent signal. Each update from the incoming signals will be used to step the state forward. The outgoing signal represents the current state. 13

Mario example I  main : Signal Element main = Signal.map2 view Window.dimensions (Signal.foldp update mario input)  view : (Int, Int) -> Model -> Element  update : (Float, Keys) -> Model -> Model 14

Mario example II  main : Signal Element main = Signal.map2 view Window.dimensions (Signal.foldp update mario input)  input : Signal (Float, Keys) input = let delta = Signal.map (\t -> t/20) (fps 30) in Signal.sampleOn delta (Signal.map2 (,) delta Keyboard.arrows) 15

Modified Mario I  I modified the Mario example to also accept mouse position signals  main : Signal Element main = Signal.map3 view Window.dimension Mouse.position (Signal.foldp update mario input)  view : (Int, Int) -> (Int, Int) -> Model -> Element  update : (Float, Keys) -> Model -> Model 16

Modified Mario II  main : Signal Element main = Signal.map3 view Window.dimension Mouse.position (Signal.foldp update mario input)  input : Signal (Float, Keys, Location) input = let delta = Signal.map (\t -> t/20) (fps 30) in Signal.sampleOn delta (Signal.map3 (,,) delta Keyboard.arrows Mouse.position) 17

Modified Mario III  type alias Model = { x : Float, y : Float, vx : Float, vy : Float, dir : Direction, mouse : (Int, Int) }  update : (Float, Keys, Location) -> Model -> Model update (dt, keys, position) mario = mario |> gravity dt |> jump keys |> walk keys |> physics dt |> attract position -- defined as type alias Location = (Int, Int) 18

Questions  main : Signal Element main = Signal.map3 view Window.dimension Mouse.position (Signal.foldp update mario input) input : Signal (Float, Keys, Location) input = let delta = Signal.map (\t -> t/20) (fps 30) in Signal.sampleOn delta (Signal.map3 (,,) delta Keyboard.arrows Mouse.position)  Why isn’t Window.dimension handled in input ?  Why do I need to mention Mouse.position twice? 19

Collage example  A Collage is used to display things on the screen  view : (Int, Int) -> (Int, Int) -> Model -> Element view (w',h') mario = let -- many definitions omitted: w, h, marioImage, etc. in collage w' h' [ rect w h |> filled (rgb ), rect w 50 |> filled (rgb ) |> move (0, 24 - h/2), marioImage |> toForm |> move position ] 20

21 The End