Enum-FP

Functional Enum type / Sum type for javascript with simple pattern matching

Checkout the docs for more information

Medium article on SumTypes using EnumFP

Install

To add the project to your project

yarn add enum-fp

Usage

Import it to your file

import Enum from 'enum-fp';

Create an enum type

const Action = Enum([ 'Add', 'Edit', 'Delete', 'Get' ]);

// Or with a fixed number of arguments
const Maybe = Enum({
    Just: [ 'value' ],
    Nothing: [],
});

Create an instance of the type using one of the contructors

const action = Action.Edit(2, 'Hello world and India');

Pattern matching

const Action = Enum([ 'Add', 'Edit', 'Delete', 'DeleteAll', 'Get' ]);

const logMessage = action => console.log('>>', 
  Action.match(action, {
    Edit: (id, message) => `Editing [${id}] to "${message}"`,
    Add: message => `Adding "${message}"`,
    Delete: id => `Deleting [${id}]`,
    DeleteAll: () => 'Deleting all entries',
    _: () => 'Unknown action', // To handle default cases, use _
  })
);

logMessage(Action.Add('Earth'));      // >> Adding "Earth"
logMessage(Action.Add('Earth 2'));    // >> Adding "Earth 2"
logMessage(Action.Add('Pluto'));
logMessage(Action.Add('Pluto'));       // >> Adding "Pluto1"
logMessage(Action.Edit(1, 'Mars'));   // >> Editing [2] to "Mars"
logMessage(Action.Delete(2));         // >> Deleting [3]
logMessage(Action.Add('Pluto'));      // >> Adding "Pluto"
logMessage(Action.DeleteAll());       // >> Deleting all entries

// As Get action is not handled in the pattern, it will execute the default
logMessage(Action.Get());             // >> Unknown action

Type validation

You can add strict type validation instead of argument descriptions. You can read more about types module here

import T from 'enum-fp/types';

const TodoAction = Enum({
  Add: [ T.String('message') ],
  SetChecked: [ T.Number('id'), T.Bool('isChecked') ],
  Delete: [ T.Number('id') ],
  Edit: [ T.Number('id'), T.String('message') ],
  DeleteAll: [],
});

NOTE: The string passed to the functions are just for documentation purposes and are optional. It won't affect the behavior of the type in any way.

Enum use cases

In the react world

You can use it to manage react component state! Checkout the documentation

Safely work with empty/invalid states

• Working with invalid values

// Just an example. You should use `Maybe` functor in cases like these
const Value = Enum({ Invalid: [], Valid: ['value'] });

const extractName = user => user && user.name
  ? Value.Valid(user.name)
  : Value.Invalid();

const splitBySpace = Value.cata({
  Valid: name => name.split(' '),
  Invalid: () => [],
});

const getNameSplit = compose(splitBySpace, extractName);

const [ firstName, lastName ] = getNameSplit({ name: 'Akshay Nair' }); // >> returns ['Akshay','Nair']

In the functional world

If you are unfamiliar with functors, you can read Functors in JS blog post.

• Maybe

Maybe functor is used to handle null.

const Maybe = Enum({ Just: ['value'], Nothing: [] });

const fmap = fn => Maybe.cata({
  Just: compose(Maybe.Just, fn),
  Nothing: Maybe.Nothing,
});

• Either

Either functor is used for handling exceptions

const Either = Enum({ Left: ['error'], Right: ['value'] });

const fmap = fn => Either.cata({
  Left: Either.Left,
  Right: compose(Either.Right, fn),
});
const fmapFail = fn => Either.cata({
  Left: compose(Either.Left, fn),
  Right: Either.Right,
});