Overcoming Software

Elm Architecture in PureScript IV: Effects

The Final Chapter-ing

In the last post, I covered higher order components and making dynamic lists of components. We’re going to get into effects and AJAXing with this. It’s almost entirely like you might expect, given the previous posts, but we’ll finally start to specialize that g functor!

As always, the code is available in this repository

Gif loader!

I’m pretty excited for this. Counters are boring and now I can get cat gifs delivered? This is precisely what I want from all web UIs, really.

So, we’re going to have a topic and a URL for the current gif. The only input we’ll need to handle is requesting a new gif. We’ll be interacting with the giphy public API to do this.

Let’s define our state and inputs:

-- src/Example/Five.purs

type State =
  { topic :: String
  , gifUrl :: String

initialState :: State
initialState = { topic: "cats", gifUrl: "" }

data Input a
  = RequestMore a

STANDARD SUPER BORING, you knew it was coming. Since we know we’ll be using effects, we’ll also define a type for the effects our component will be using:

type GifEffects eff = HalogenEffects (ajax :: AJAX | eff)

Halogen defines a type of effects that it normally uses, and we’re just adding the AJAX effect on top of that.

The ui component is pretty standard. We’ve replaced the g functor with Aff (GifEffects ()) to indicate that we’ll be using the asynchronous effects monad. The render function is boring, so we’ll get right to the eval function.

ui :: Component State Input (Aff (GifEffects ()))
ui = component render eval
    render :: -- *yawn* let's skip to eval

    eval :: Eval Input State Input (Aff (GifEffects ()))
    eval (RequestMore a) = do
      state <- get
      newGifUrlFn <- liftFI (fetchGif state.topic)
      modify \s -> s { gifUrl = newGifUrlFn s.gifUrl }
      pure a

liftFI is a function that lifts effects from the free monad. So we can phone home, launch missiles, write to the console, or do AJAX all from the liftFI function. Well, to be precise, we can only do those things if they’re included in the Aff (GifEffects ()) effects type! (I haven’t checked HalogenEffects…)

fetchGif uses the Affjax library to make the request, read the JSON, and return either a function to transform the current URL to the new one, or a function that doesn’t change it at all.

fetchGif :: forall eff. String -> Aff (ajax :: AJAX | eff) (String -> String)
fetchGif topic = do
    result <- AJ.get (giphyRequestUrl topic)
    let url = readProp "data" result.response >>= readProp "image_url"
    pure (either (flip const) const url)

So if we get a Left value out of the URL, then we do flip const on the Left value, and then finally on the URL in the state. If the request succeeds, then we do const result over the old URL, which sets it to be equal to the result.

readProp tries to read the JSON property of the object passed, and either returns the result or a Left error type if it wasn’t successful. That can be a quick way of dealing with data if you don’t want to write a full JSON parser.

And that’s it! We’ve got effects. NBD. Running the code in main looks the same as we’d expect:

runEx5 = runUI Ex5.ui Ex5.initialState

Multi Gif Loaders?!

Alright, how about a pair of gif loaders? This is very similar to the pair of counters we had in two, but we don’t need to worry about resetting them.

In fact, the entire bit of code (imports and all!) is 28 lines!

module Example.Six where

import Prelude
import Control.Plus (Plus)
import Data.Functor.Coproduct (Coproduct(..))
import Control.Monad.Aff (Aff())

import Halogen
import qualified Halogen.HTML.Indexed as H

import qualified Example.Five as Gif

data Input a = NoOp a

type State =
  InstalledState Unit Gif.State Input Gif.Input (Aff (Gif.GifEffects ())) Boolean

type Query =
  Coproduct Input (ChildF Boolean Gif.Input)

ui :: Component State Query (Aff (Gif.GifEffects ()))
ui = parentComponent render eval
    render _ =
        [ H.slot true \_ -> { component: Gif.ui, initialState: Gif.initialState }
        , H.slot false \_ -> { component: Gif.ui, initialState: Gif.initialState }
    eval :: EvalParent Input Unit Gif.State Input Gif.Input (Aff (Gif.GifEffects ())) Boolean
    eval (NoOp a) = pure a

I’m using Boolean as the slot type because it naturally only has two elements, and any type that just has two elements is equivalent to boolean, and this way I don’t have to make ord/eq instances…

List of Gifs

Next up is a list of gif downloaders. But wait. Instead of making a list of gif downloaders, let’s just make another higher order component that contains a list of other components.

We’ll model it off of Example.Three, so much of the code should look pretty familiar. First we’ll need to define state, query, child slots, etc…

type StateP =
  { itemArray :: Array Int
  , nextID :: Int

initialStateP :: StateP
initialStateP =
  { itemArray: []
  , nextID: 0

data QueryP a
  = AddItem a
  | RemItem a

newtype Slot = Slot Int

We use the P suffix because we’ll want to create type synonyms for the installed state and child query stuff.

The Slot type needs an instance of the Eq and Ord type classes. Fortunately, the newer versions of PureScript include a mechanism for generically deriving these. We have to import Data.Generic, and then we get to do:

derive instance genericSlot :: Generic Slot

instance ordSlot :: Ord Slot where
  compare = gCompare

instance eqSlot :: Eq Slot where
  eq = gEq

Nice! Much less tedious than writing the instances out manually. (Here’s hoping that deriving (Eq, Ord) makes it into the language soon…)

Now we’ll define the listUI. Like we did with the higher-order “add a remove button” component, we’ll use two type variables for the child state and child query.

makeList :: forall g p s f. (Plus g)
         => Component s f g
         -> s
         -> Component (State s f g) (Query f) g
makeList comp initState = parentComponent render eval
    render state =
        [ H.button [ E.onClick $ E.input_ AddItem ]
                   [ H.text "+" ]
        , H.button [ E.onClick $ E.input_ RemItem ]
                   [ H.text "-" ]
        , H.ul_ (map (\i -> H.slot (Slot i) (initComp comp initState)) state.itemArray)

    initComp :: Component s f g -> s -> Unit -> { component :: _, initialState :: _ }
    initComp c s _ = {component: c, initialState: s}

    eval :: EvalParent QueryP StateP s QueryP f g Slot
    eval (AddItem next) = modify addItem $> next
    eval (RemItem next) = modify remItem $> next

The only new thing about this is the $> operator, but it does what you’d expect given it’s place in the function.

And we’re done with the component definition! Let’s run it and see where we go:

-- src/Main.purs
runEx7 = runUI (Ex7.makeList Ex5.ui Ex5.initialState) Ex7.initialState

We don’t even need a type signature. Nice!

And thus concludes my tutorial series on the Elm Architecture in PureScript. I’m not going to cover animation because I don’t know how it works, and that’s beyond the scope of the Halogen framework.

  1. Elm vs PureScript I: War of the Hello, Worlds
  2. Elm vs PureScript II
  3. Elm Architecture in PureScript III: Dynamic Lists of Counters