A simple application state store in native PureScript, inspired by Redux (and compatible with the Redux Dev Tools extension).

The store is built around the idea of actions (usually an algebraic datatype like data Action = DoThisThing | DoTheOtherThing) and an update function. The update function, a pure function, is responsible for updating the application state when you trigger an action using the dispatch function. To respond to these state changes, you subscribe to the store with an effectful function that updates the environment to reflect the new state.

createStore :: ∀ e s a. (Generic a, Generic s) ⇒ (a → s → s) → s → Eff (store :: STORE | e) (Store e a s)

You create a store using the createStore function, which takes an update function and a starting state. The Store you get back has two properties store.dispatch and store.subscribe, as described above.

type Store e a s =
  { subscribe :: (s → Eff e Unit) → Eff (store :: STORE | e) Unit
  , dispatch :: a → Eff (store :: STORE | e) Unit
  }

You'll note from looking at createStore that your actions and your state must both have Generic instances. This is in order to work nicely with the Redux Dev Tools extension, which provides features like being able to export states as JSON files and import them back into a running application. You can usually get the compiler to generate these by using generic deriving, for instance derive instance genericAction :: Generic Action.

A simple working example, where the application state is just an integer, with two actions which increment and decrement it:

module Main where

import Data.Generic (class Generic)
import Control.Monad.Eff.Console (log)

import Data.Store (createStore)

type State = Int

seedState :: State
seedState = 0

data Action = Pred | Succ
derive instance genericAction :: Generic Action

update :: Action → State → State
update Pred n = n - 1
update Succ n = n + 1

main = do
  store ← createStore update seedState

  store.subscribe \n → log ("The number is " <> show n)
  -- prints "The number is 0" to the console.

  store.dispatch Succ
  -- prints "The number is 1" to the console.

  store.dispatch Pred
  -- prints "The number is 0" to the console.

Copyright 2016 Bodil Stokke

This program is free software: you can redistribute it and/or modify it under the terms of the GNU Lesser General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version.

This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Lesser General Public License for more details.

You should have received a copy of the GNU Lesser General Public License along with this program. If not, see http://www.gnu.org/licenses/.