Combine Redux action handlers into a type-safe reducer.
bower install purescript-aspen
First write some action handlers. An action handler is a function that handles
one and only one Redux action. An Action is of type Action (t :: Symbol) p
where t is the type of the action and p is the type of the payload. You
can write as many of these as you want. In this example I have 3.
addTask :: State -> Action "ADD_TASK" Task -> State
addTask = ...
toggleCompleteness :: State -> Action "TOGGLE_COMPLETENESS" Id -> State
toggleCompleteness = ...
removeTask :: State -> Action "REMOVE_TASK" Id -> State
removeTask = ...
After we have all of the action handlers, we need to combine them into a reducer
using createReducer, handle, and >>=>>. This creates a reducer with the
type of all of the supported actions in the signature. This provides compile
time safety. All unsupported actions types will not change the state.
reducer
:: forall p t r1 r2
. RowCons t (Action t p) r1
( "REMOVE_TASK" :: Action "REMOVE_TASK" Id
, "TOGGLE_COMPLETENESS" :: Action "ADD_TASK" Id
, "ADD_TASK" :: Action "ADD_TASK" Task
| r2)
=> IsSymbol t
=> State -> Action t p -> State
reducer = createReducer $
handle addTask
>>=>> toggleCompleteness
>>=>> removeTask
To see a more detailed tutorial checkout the aspen test.
Writing reducers in Purescript is annoying, if done naïvely. All the type
un-safety of JavaScript can reflect in the type of the reducer and branching
on action.type is annoying. However, we are guaranteed to be immutable and
have zero side-effects! These are the two reducer "rules" that are not enforced
in JavaScript. If we could just get rid of the problems, life would be great.
With aspen, you can have a great life.
Aspen allows you to combine action handlers into a single Redux reducer. Each handled action is reflected in the type of the reducer making it easy to see what each reducer handles at-a-glance. Since the Actions are typed, you cannot have the wrong payload. Doing so, would result in a compile time error! Actions unknown to a reducer are passed through and ignored.
This library is like redux-actions, but PureScript-ian. Thank you for the inspiration.
- Module documentation is published on Pursuit.