Boilerplate-free functional sum types in TypeScript
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lorefnon and pelotom Clarify default behavior of value tag (#49)
Add notes on the default behavior when value tag is not specified, which can be non-intuitive.
Latest commit 4da1790 Oct 31, 2018

README.md

Unionize Build Status Coverage Status

Define unions via records for great good!

Installation

yarn add unionize

Example

Provide unionize with a mapping of tags to value types:

import { unionize, ofType } from 'unionize';

const Actions = unionize({
  ADD_TODO: ofType<{ id: string; text: string }>(),
  CLEAR_TODOS: {}, // For "empty" types, just use {}
});

Extract the inferred tagged union:

import { UnionOf } from 'unionize'

type Action = UnionOf<typeof Actions>;

The inferred type is:

type Action = ({
    tag: "ADD_TODO";
} & {
    id: string;
    text: string;
}) | {
    tag: "CLEAR_TODOS";
}

We can also optionally provide tag and/or value property names:

import { unionize, ofType } from 'unionize'

const Actions = unionize({
  ADD_TODO: ofType<{ id: string; text: string }>(),
  SET_VISIBILITY_FILTER: ofType<'SHOW_ALL' | 'SHOW_ACTIVE' | 'SHOW_COMPLETED'>(),
  TOGGLE_TODO: ofType<{ id: string }>(),
  CLEAR_TODOS: {}, // For "empty" types, just use {}
}, {
    tag:'type',
    value:'payload',
});

Now the inferred type would be:

type Action =
  | { type: 'ADD_TODO'; payload: { id: string; text: string } }
  | { type: 'SET_VISIBILITY_FILTER'; payload: 'SHOW_ALL' | 'SHOW_ACTIVE' | 'SHOW_COMPLETED' }
  | { type: 'TOGGLE_TODO'; payload: { id: string } }
  | { type: 'CLEAR_TODOS'; payload: {} };

which is now FSA compliant.

Note that when the tag property is omitted, "tag" will be used as the tag property. But if the value tag name is omitted, the generated sum type does not use a default "value" property -- Instead, it intersects the value type with { [tagProp]: tag }.

While this is convenient when dealing with value types which are objects (No need to access the value through a .value accessor), it should be obvious that the value type can only be an object type if value tag is omitted.

So something like ofType<'SHOW_ALL' | 'SHOW_ACTIVE' | 'SHOW_COMPLETED'> is only legal as a value type when a value property name is provided:

const Actions = unionize({
  ADD_TODO: ofType<{ id: string; text: string }>(),
  SET_VISIBILITY_FILTER: ofType<                           
    "SHOW_ALL" | "SHOW_ACTIVE" | "SHOW_COMPLETED"
  >()
})

The above fails with the error

Type '"SHOW_ALL" | "SHOW_ACTIVE" | "SHOW_COMPLETED"' is not assignable to type '{ [tag: string]: any; }'.
  Type '"SHOW_ALL"' is not assignable to type '{ [tag: string]: any; }'.

Having done that, you now have at your disposal:

Element factories

store.dispatch(Actions.ADD_TODO({ id: 'c819bbc1', text: 'Take out the trash' }));
store.dispatch(Actions.SET_VISIBILITY_FILTER('SHOW_COMPLETED'));
store.dispatch(Actions.CLEAR_TODOS()); // no argument required if value type is {}

Match expressions

const todosReducer = (state: Todo[] = [], action: Action) =>
  Actions.match(action, {
    // handle cases as pure functions instead of switch statements
    ADD_TODO: ({ id, text }) => [...state, { id, text, completed: false }],
    TOGGLE_TODO: ({ id }) =>
      state.map(todo => todo.id === id
        ? { ...todo, completed: !todo.completed }
        : todo
      ),
    // handles the rest; if not provided, cases must be exhaustive 
    default: a => state // a === action. Useful for curried version
  });

action can be omitted; in that case the result of match is a function:

const getIdFromAction = Actions.match({
  ADD_TODO: ({ id}) => id,
  TOGGLE_TODO: ({ id }) => id,
  default: a => { throw new Error(`Action type ${a.type} does not have an associated id`); },
});

const action = Actions.ADD_TODO({ id: 'c819bbc1', text: 'Take out the trash' });
const id = getIdFromAction(action); // id === 'c819bbc1'

Type guards

const epic = (action$: Observable<Action>) => action$
  .filter(Actions.is.ADD_TODO)
  // The appropriately narrowed type of the resulting observable is inferred...
  .mergeMap(({ payload }) => console.log(payload.text));

Type casts

const { id, text } = Actions.as.ADD_TODO(someAction); // throws if someAction is not an ADD_TODO

Transform expressions

transform is a shorthand alternative to match for when you are converting from the union type to itself, and only want to handle a subset of the cases, leaving the rest unchanged:

const Light = unionize({ On: ofType<{ percentage: number }>(), Off: {} });

const turnOn = Light.transform({ Off: () => Light.On({ percentage: 100 }) });
const dim = Light.transform({ On: prev => Light.On({ percentage: prev.percentage / 2 }) });

const off = Light.Off();
const dimmed = dim(off); //didn't match. so dimmed === off
const on = turnOn(off);

// can accept an object right away
const toggled = Light.transform(on, {
  On: () => Light.Off(),
  Off: () => Light.On({ percentage: 50 }),
});

Breaking changes from 1.0.1

config object

Now unionize accepts an optional config object instead of two additional arguments.

// before
unionize({...}, 'myTag', 'myPayloadProp');
unionize({...}, 'myTag');

// after
unionize({...}, { tag:'myTag', value:'myPayloadProp' });
unionize({...}, { tag:'myTag' });
unionize({...}, { value:'myPayloadProp' }); // <-- previously not possible

match

Whereas previously match was always curried, now it can alternatively accept the object to match as a first argument. Additionally, the default case is now expressed as just another property in the cases object.

// before
Light.match({
  On: () => 'is on'
}, () => 'is off'
)(light);

// after
Light.match({
  On: () => 'is on',
  default: () =>'is off'
})(light);
Light.match(light, {
  On: () => 'is on',
  default: () => 'is off',
});