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PureScript client-side MVC frameworks

PureScript as a full language replacement for Elm

Elm is a client side functional programming framework that has made a trend in Web client frameworks. With the aim of making things easier it has dropped a nice functional reactive programming model and has adopted a Model-View-Controller architecture as the functional blocks that the object-oriented reactJS system offers. But its language has no polymorphism for containers (* -> *), and one for simple types called "type categories" which values (number, comparable, appendable) are hardwired in the compiler, making it a rather limited subset of what the language it is inspired in (Haskell) offers.

See Elm from a Haskell perspective.

PureScript offers alternatives for designing Web User Interfaces much nearer to the language power of Haskell and much lighter weight than GHCJS (no GHC RTS emulation).

See also PureScript introduction and recursively defined lazy lists.

PureScript Pux

Here is an example from the Pux web, of an uncommon multi-URL example that simulates routing. Instead of fetching URL's by HTTP, it decodes them to an Event type with a Route, which changes the State's currentRoute in the controller and renders it in the View, pushing the target URL into the navigator's history stack, and also turns history pops (Back button presses) in Events in an Elm's subscription like manner. I have reordered the snippets and indented some parts, to highlight the MVC constituents:

{-| MVC highlights of the Pux multi-page (multi-URL) sample app.
    * Here the Language is PureScript !!! 
    * Square bracket literals denote type Array -}

-- the model
data Route = Home | Users String | User Int | NotFound  -- the pages
type State = {currentRoute :: Route}

-- the message
data Event = PageView Route | Navigate String {- the URL -} DOMEvent

-- the view
view :: State -> Html Event
view state = div do
                navigation
                page state.currentRoute
    where
        -- content
        page :: Route -> Html Event
        page Home           = h1 $ text "Home"
        page (Users sortBy) = h1 $ text ("Users sorted by:" <> sortBy)
        page (User id)      = h1 $ text ("User: " <> show id)
        page NotFound       = h1 $ text "Not Found"

        navigation :: Html Event
        navigation =
          nav do
            ul
              li $ a ! href "/" #! onClick (Navigate "/") $ text "Home"
              li $ a ! href "/users" #! onClick (Navigate "/users") $ text "Users"
              li $ a ! href "/users?sortBy=age" #! onClick (Navigate "/users?sortBy=age") $ text "Users sorted by age."
              li $ a ! href "/users/123" #! onClick (Navigate "/users/123") $ text "User 123"
              li $ a ! href "/foobar" #! onClick (Navigate "/foobar") $ text "Not found"
              
-- the URL to (PageView Route) decoder
match :: String -> Event
match url = PageView $ fromMaybe NotFound $ router url $
  Home <$ end
  <|>
  Users <$> (lit "users" *> param "sortBy") <* end
  <|>
  Users "name" <$ (lit "users") <* end
  <|>
  User <$> (lit "users" *> int) <* end    

-- the controller
foldp :: ∀ fx. Event -> State -> EffModel State Event (history :: HISTORY, 
                                                       dom :: DOM | fx)
foldp (PageView route) st =  noEffects $ st { currentRoute = route }
foldp (Navigate url ev) st =
  onlyEffects st [ liftEff do
                     preventDefault ev
                     
                     -- push url and title into the Navigator history
                     h <- history =<< window
                     pushState (toForeign {}) (DocumentTitle "") (URL url) h
                     
                     -- emit a PageView message 
                     -- `match` decodes the URL into a (PageView Route) Event message
                     pure $ Just $ match url
                 ]

main = do
  -- sampleURL emits a Signal on navigator history pops (as a result of the Back button)
  urlSignal <- sampleURL =<< window
  let routeSignal = urlSignal ~> match  -- Signal to PageView Event "natural transf."
  app <- start
    { initialState: { currentRoute: Home }
    , view
    , foldp
    , inputs: [routeSignal]
    }

In simpler apps the rendering setup would tell whether to render

  • to a browser DOM Element, with renderToDOM
  • to console output, with renderToString

PureScript Thermite

  • Thermite is a Purescript-react framework that offers some component combination possibilities, each with its model state, event messages and controller.

    1. you may combine several components of equal model and message types into an app, with the component Semigroup operator (<>).

      • to combine components with different models, there are combinators that let you focus with a lens, the specific model type from the group model product type, or a prism in case of optional components.

      • to combine components with different messages (called actions), there are combinators that let you focus with a prism the specific message type from the group message sum type, but sharing the same message type enables mutual interaction.

      Combining independent components:

       spec1 :: Spec _ S1 _ A1
       spec2 :: Spec _ S2 _ A2
       --
       groupSpec :: Spec _ (Tuple S1 S2) _ (Either A1 A2)
       groupSpec = focus _1 _Left spec1 <> focus _2 _Right spec2
    2. you may also build a group of equally typed components as a List, where the state is a List of the states, and the message (called action) is indexed with the index of the originating subcomponent in the List.

PureScript Halogen

  • Halogen is a somewhat more complex Purescript framework.

    • A component may include slots for subcomponents in the HTML DSL, enabling bidirectional parent-child communication reflected in its generator parameters:

      • A value of type Slot (the slot address) is used as slot identifier and makes possible to send synchronous request queries from within the controller, to the slot subcomponent.

      • Slot "input" values (at the slot's input parameter) are a means of passing values into a child component every time a parent re-renders, to be handled through the subcomponent receiver field "input" handler where you may tag the incoming message to be processed by the subcomponent controller

      • The slot "output" handler parameter: a subcomponent may raise messages to its parent container component to be handled through the slot's "output" handler where you may tag the incoming message to be processed by the component controller

    • The component state, instead of given as parameter to the eval controller, it is held in the controller's MonadState monad.

    • There is a weird thing in the way messages, called Queries, are defined, because the system enforces the return value of the eval controller to be determined by the query message content or, in case of request queries, the result of its callback component, which has a strange type of (queryResult -> a) (see Query evaluation). This is enforced by defining the controller type as a natural transformation: an homomorphism between structures, the query message and the controller monad, threading some obscure value through them.