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Combine Redux action handlers into a type-safe reducer.


bower install purescript-aspen

Quick start

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.

  :: forall p t r1 r2
  .  RowCons t (Action t p) r1
      ( "REMOVE_TASK" :: Action "REMOVE_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.