Simple Routing

Purescript Halogen

Let’s look at routing in Halogen. We will start by implementing a router ourselves to show that just because we are using Halogen it doesn’t mean we can’t use the Html api as well.

We need an app that informs us of the best variety of potato depending on how we want to cook them. A vital quality of life tool.



First setup the project as per here.

We alse want to install the web-html and web-events packages to give us access to the Html api. So in the command line cd to the project directory and:

> spago install web-html web-events
Installation complete.

The router

Lets start by creating the router functionality. Create a new file in src called Router.purs. Add the following imports :

module Router where

import Prelude

import Data.Maybe (Maybe(..))
import Effect (Effect)
import Web.Event.EventTarget as EventTarget
import Web.HTML (window)
import Web.HTML.Event.HashChangeEvent.EventTypes as ET
import Web.HTML.Location as Location
import Web.HTML.Window as Window

We want a simple type to represent our routes :

data Route = Roast | Chips | Salad


Then lets create a function that will take the hash from the location url. The hash is the part of the url that follows the # - for example Changing the hash of the url in the browser will not cause a page reload, but the change will be stored in the browsers history. It is ideal to use for routing with single page applications.

If it is a matching Url it will return the Route, otherwise it returns Nothing.

getRoute :: Effect (Maybe Route)
getRoute = do
  hash <- window >>= Window.location >>= Location.hash 
  pure $ case hash of
           "#roast" -> Just Roast
           "#chips" -> Just Chips
           "#salad" -> Just Salad
           _ -> Nothing

Note that this is running in the Effect monad. Just about everything with the Html api runs in this monad since you can create side effects all over the place!

The first line retrieves the hash from the location from the window.

  hash <- window >>= Window.location >>= Location.hash 

Each function is running in the Effect monad, so we can’t get the actual value directly - we have to stay in Effect. We could rewrite this as:

window' <- window
location <- Window.location window'
hash <- Location.hash location

But that is a bit long winded, so we can take advantage of bind as per here.

Next we match on the hash string to get our Route. Obviously this is a bit naff for anthing more than the most simple of routes, but it will do for now.


Our router will work by listening to the HashChange event of the browser. The router function hooks into this event :

router :: (Route -> Effect Unit) -> Effect Unit
router cb = do
  listener <- EventTarget.eventListener $ const matchRoute
  window' <- window 
  EventTarget.addEventListener ET.hashchange listener false $ Window.toEventTarget window'
    matchRoute = do
              route <- getRoute
              case route of
                Just route' -> cb route'
                Nothing -> pure unit

The parameter passed to the function has type Route -> Effect Unit. This is the callback that will be invoked when the route changes. Note it also runs in Effect.

First let us setup the callback that will be invoked by the event.

  listener <- EventTarget.eventListener $ const matchRoute

eventListener takes a callback function and returns an EventListener - all within Effect. The callback function we are passing is const matchRoute. We don’t need the event parameter so we ignore it using const.

    matchRoute = do
              route <- getRoute
              case route of
                Just route' -> cb route'
                Nothing -> pure unit

This function will get the route and if it is a valid route it will invoke the main callback with the route as a parameter. Otherwise it does nothing.

Now we set up the listener:

  window' <- window 
  EventTarget.addEventListener ET.hashchange listener false $ Window.toEventTarget window'

We neet to pull the window out of Effect into the window’ variable. Then we call addEventListener to set up the HashChangeEvent. The browser will now call matchRoute whenever there is a hashchange event.


Each page is a separate Halogen component. The details aren’t particularly important, but you can pull down the code from here.


Now lets setup the parent component that will handle showing each page according to the route. A lot of this should be familiar from the last time.

Create a new file under src, Parent.purs. Add the following imports:

module Parent where

import Prelude

import Children as Children
import Data.Maybe (Maybe(..))
import Data.Symbol (SProxy(..))
import Halogen as H
import Halogen.HTML as HH
import Halogen.HTML.Properties as HP
import Router as R

We are going to need to create our child slots, one for each page:

type ChildSlots = ( roast :: Children.Slot Unit
                  , chips :: Children.Slot Unit
                  , salad :: Children.Slot Unit

_roast :: SProxy "roast"
_roast = SProxy

_chips :: SProxy "chips"
_chips = SProxy

_salad :: SProxy "salad"
_salad = SProxy

Our State contains the current route :

type State = { currentRoute :: R.Route }

This component will get notifications about the current route and route changes from the parent, in this case that is going to be the main function. So we need to set up our Query and Input types to receive the routes.

data Query a = ChangeRoute R.Route a
type Input = R.Route

Setup the page component:

page :: forall m. H.Component HH.HTML Query Input Void m
page = H.mkComponent { initialState
                     , render
                     , eval : H.mkEval $ H.defaultEval
                                { handleQuery = handleQuery }

Our initialState function requires that we are passed the initial route to display.

initialState :: Input -> State
initialState route = { currentRoute : route }

The Query sends us a new route to display. We modify our state to hold this new route.

handleQuery :: forall m a. Query a -> H.HalogenM State Void ChildSlots Void m (Maybe a)
handleQuery (ChangeRoute route k) = do
    H.modify_ _{ currentRoute = route }
    pure $ Just k

Now we move on to the rendering. First lets render the menu :

renderMenu :: forall m . H.ComponentHTML Void ChildSlots m
renderMenu = HH.div_ [ HH.a [ HP.href "#roast" ]
                                [ HH.text "Roast" ]
                     , HH.text " | "
                     , HH.a [ HP.href "#chips" ]
                                [ HH.text "Chips" ]
                     , HH.text " | "
                     , HH.a [ HP.href "#salad" ]
                                [ HH.text "Salad" ]

Then we render the current route :

renderPane :: forall m . State -> H.ComponentHTML Void ChildSlots m
renderPane state = 
  HH.div_ [ case state.currentRoute of
              R.Roast -> HH.slot _roast unit Children.roast unit absurd
              R.Chips -> HH.slot _chips unit Children.chips unit absurd
              R.Salad -> HH.slot _salad unit Children.salad unit absurd

We render a different slot based on what our current route is.

Finally we render the whole app, the menu followed by the current page:

render :: forall m . State -> H.ComponentHTML Void ChildSlots m
render state =
  HH.div_ [ renderMenu
          , renderPane state


The main function is where we will create our component and link it to our router event listener. In src/Main.purs set the following imports :

import Data.Maybe as Maybe
import Effect (Effect)
import Effect.Aff (launchAff_)
import Halogen as H
import Halogen.Aff as HA
import Halogen.VDom.Driver (runUI)
import Parent as Parent
import Router as R

Then change the main to the following :

main :: Effect Unit
main = HA.runHalogenAff do
         body <- HA.awaitBody
         current <- H.liftEffect $ Maybe.fromMaybe R.Roast <$> R.getRoute
         io <- runUI current body

           changeRoute route = launchAff_ $
                               io.query $
                               H.tell $
                               Parent.ChangeRoute route

         H.liftEffect $ R.router changeRoute

Lines of interest:

        current <- H.liftEffect $ Maybe.fromMaybe R.Roast <$> R.getRoute

We need to call R.getRoute to get the current route so we know which page to display initially. This may just return Nothing, so we use Maybe.fromMaybe to default to the R.Roast route if needed. We are using <$> here since R.getRoute returns an Effect (Maybe Route) and we want it to be an Effect Route. Using <$> allows us to do the work (call fromMaybe) from within Effect.

Finally, R.getRoute returns the route in the Effect monad. But Halogen runs in the Aff monad (an asynchronous layer over Effect). To move from the Effect monad into Aff we need to call H.liftEffect.

Next we call runUI to kick off the whole halogen process. This time we capture the return of runUI. This is a reference to our parent component. We will use this to send it ChangeRoute messages.

We setup the callback function to pass into our router:

           changeRoute route = launchAff_ $
                               io.query $
                               H.tell $
                               Parent.ChangeRoute route

The io variable we recieved from runUI has a query function property that we can use to send queries to the component. We need to make our query a tell query (one that doesn’t require any result) using H.tell. Our message is Parent.ChangeRoute.

io.query is being run in the Aff monad, but the router expects the callback to be run in the Effect monad. launchAff_ will do this for us.

Finally, we call our router and pass the changeRoute callback to it :

         H.liftEffect $ R.router changeRoute

Again, we need to move from Effect to Aff with H.liftEffect.


That covers some very simple routing. Next I’ll look into using the routing library that handles much more complex cases.

You can find sample code here.