algebraic-type

Algebraic types for JavaScript. Inspired by adt and the Elm architecture, but works with plain object.

Why?

To reduce boilerplate in generating and checking Redux action objects. To reduce errors when creating Redux action types and reducers. To allow action’s shape to be specified.

Examples

import AlgebraicType from 'algebraic-type'

Simple Example

type Action = Increment | Decrement

const Action = AlgebraicType({
  Increment: { },
  Decrement: { },
})

Todo Example from Redux

• The Flux/Redux approach is to define action types as string constants. Each action type name is spelled out twice. There is a possibility that the constant or the value is misspelled.

  export const ADD_TODO = 'ADD_TODO'
  export const DELETE_TODO = 'DELETE_TODO'
  export const EDIT_TODO = 'EDIT_TODO'
  export const COMPLETE_TODO = 'COMPLETE_TODO'
  export const COMPLETE_ALL = 'COMPLETE_ALL'
  export const CLEAR_COMPLETED = 'CLEAR_COMPLETED'

• In Elm, you declare your action as an algebraic data type. This also lets you specify the shape of your action, as well as providing type safety.

  type Action
      = AddTodo String
      | DeleteTodo Int
      | EditTodo Int String
      | CompleteTodo Int
      | CompleteAll
      | ClearCompleted

• With algebraic-type, you create an algebraic type like this. Each key is the action type’s name. Inside it, you describe the shape of the object.

  const Action = AlgebraicType({
    AddTodo: {
      text: String,
    },
    DeleteTodo: {
      id: Number,
    },
    EditTodo: {
      id: Number,
      text: String,
    },
    CompleteTodo: {
      id: Number,
    },
    CompleteAll: { },
    ClearCompleted: { },
  })

  You still have the benefit of seeing all the actions in your application in a single place.

• Plain Redux Action Creators: Create an object literal directly.

  export function addTodo(text) {
    return { type: types.ADD_TODO, text }
  }

  There is a possibility that you misspelt the property name. Maybe it’s late at night and you’re hungry and thinking about some tofu soup. You typed in types.ADD_TOFU (or just ADD_TOFU in case of ES6 imports). You end up dispatching an undefined action. You may also be dispatching a malformed object.

• With algebraic-type: You invoke the value constructor.

  function addTodo(text) {
    return Action.AddTodo({ text })
  }

  The value constructor validates what’s passed into it, and returns a plain, serializable object with the type property set to the constructor’s name.

  Action.AddTodo({ text: 'Learn Redux' })
  // => { type: 'AddTodo', text: 'Learn Redux' }

  You immediately get an error if you misspell it.

  Action.AddTofu({ text: 'Learn Redux' })
  // => TypeError: Action.AddTofu is not a function

  You immediately get an error if it is not in the shape you specified.

  Action.AddTodo({ task: 'Learn Redux' })
  // => Error: missing property: "text"

  algebraic-type is not a replacement for action creators; they are simply utilities that helps you creating well-formed action types and action objects. The case for action creators still holds.

• Plain Redux: Use switch statements. If you misspeelt the imported name, your reducer simply won’t process the action.

  export default function todos(state = initialState, action) {
    switch (action.type) {
      case ADD_TODO:
        ...
   
      case DELETE_TODO:
        ...
   
      default:
        return state
    }
  }

• With algebraic-type: Use the generated matcher function. If you misspelt the action name, you immediately get a TypeError.

  export default function todos(state = initialState, action) {
    if (Action.isAddTodo(action)) {
      ...
    }
    else if (Action.isDeleteTodo(action)) {
      ...
    }
    else {
      return state
    }
  }

• TK createReducer example.

Moar Feature Ideas

These are just ideas; they are not implemented yet.

• Add name prefix to generated type to prevent them from clashing. Maybe follow the ducks convention:

  const Action = AlgebraicType({
    prefix: 'my-app/widgets/',
    Load: { },
    Create: { widget: Object },
    Update: { widget: Object },
    Remove: { widget: Object },
  })

  Another example is to construct similar-looking actions:

  function AsyncAction(prefix) {
    return new AlgebraicType({
      prefix,
      Request: { },
      Success: { response: Object },
      Failure: { error: String },
    })
  }

• Allow type composition/nested actions. This allows actions to be more modular.

  Here is an example from Elm’s architecture tutorial, a list of counters:

  import { Action as CounterAction } from './counter'
   
  const Action = AlgebraicType({
    prefix: 'my-app/main/',
    Insert: { },
    Remove: { },
    Modify: { id: String, action: CounterAction },
  })

• switch() function that takes an incoming object and switches between functions based on type.

• types() function that returns an list of available keys.