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Halo is a Halogen-inspired interface for React.

It is available as a hook: useHalo; for building entire components there is: component and component_.


Module documentation is published on Pursuit.

Using with Spago

Update the additions in your packages.dhall:

let additions =
  { react-halo =
    { dependencies = [ "aff", "free", "freeap", "react-basic-hooks", "wire" ]
    , repo = ""
    , version = "v1.0.0"
  , wire =
    { dependencies = [ "aff", "filterable", "refs", "unsafe-reference" ]
    , repo = ""
    , version = "v0.5.0"

Then install with Spago:

$ spago install react-halo

What does Halo provide?

Whether you are using the hook or one of the component helpers, the main feature that Halo provides is the eval function. It looks like:

Lifecycle props action -> HaloM props state action m a

where Lifecycle is:

data Lifecycle props action
  = Initialize props    -- when the component mounts
  | Update props props  -- when the props change
  | Action action       -- when an action is dispatched
  | Finalize            -- when the component unmounts

HaloM is also a monad transformer, and so you can lift any monad m logic into HaloM. Just be aware that in order to run the logic, Halo requires that you hoist (convert) your chosen monad into Aff before returning it.


hoist :: forall props state action m m'. Functor m => (m ~> m') -> HaloM props state action m ~> HaloM props state action m'


invertReaderT x = ReaderT \env -> Halo.hoist (flip runReaderT env) x

Working with props

props :: forall props action state m. HaloM props state action m props


fireOnChange value = do
  { onChange } <- Halo.props
  onChange value

Working with state

HaloM doesn't have any special interface for reading and modifying state, instead providing an instance of MonadState for flexibility.


Subscriptions registered using these functions are automatically tracked by Halo.

subscribe :: forall props state action m. Event action -> HaloM props state action m SubscriptionId

unsubscribe :: forall m action state props. SubscriptionId -> HaloM props state action m Unit

Event comes from the Wire library.

There is also a version for subscriptions that want to unsubscribe themselves:

subscribe' :: forall m action state props. (SubscriptionId -> Event action) -> HaloM props state action m SubscriptionId

Any subscriptions that remain when the component is unmounted are automatically unsubscribed. This prevents requiring manual clean up in the Finalize lifecycle event. Also note that new subscriptions will not be created once the Finalize event has been fired.


Also provided are functions for creating and killing forks which launch processes in separate "threads" (or as useful an approximation as we can get in JavaScript):

fork :: forall m action state props. HaloM props state action m Unit -> HaloM props state action m ForkId

kill :: forall m action state props. ForkId -> HaloM props state action m Unit

Similarly to subscriptions, when the component unmounts all still-running forks will be killed. However new forks can be created during the Finalize phase but there is no way of killing them (as with Halogen).


Finally HaloM provides an instance of Parallel for converting back and forth between HaloAp, it's applicative counterpart. This allows any logic to be easily converted to run in parallel or sequentially.