tassign

This is a simple wrapper for fixed-arity, subset-typed, non-mutating Object.assign.

Why?

This came about when writing Redux reducers in TypeScript. Since reducers must not
mutate the passed in state, a common pattern is to 'set' store properties using Object.assign():

``typescript
interface IMyState {
foo: Number;
}

function myReducer(state: IMyState, action: Action) {
switch(action.type) {
case SET_FOO_ACTION:
return Object.assign({}, state, { foo: 3 });
}

return state;
}
`

This works well enough. Object.assign to an empty object produces a new object with the new value
of foo. However I wasn't happy with it from a type-enforcement perspective.

The reason is that for a reducer use case, I would like to enforce that the reducer can only set known
properties of IMyState.

Using the regular, intersection-based Object.assign typings, this is perfectly legal:

`typescript
interface IMyState {
foo: Number;
}

function myReducer(state: IMyState, action: Action) {
switch(action.type) {
case SET_FOO_ACTION:
// No TS error - I would like there to be.
return Object.assign({}, state, { foo: 3, bar: 'wat' });
}

return state;
}
`

Formally, Object.assign(t:T, u:U) returns a type which is the _intersection_ of T & U, meaning that properties
can be added onto the assignee. This is reasonable in the general case for Object.assign, but it's not what
I want in a reducer.

In a reducer, I want assign(t:T, u:U) to enforce a subset rule: the return value should be an instance of T, and only
subsets of T's type are legal values for U. In short I want to make sure you can only assign members of U to the
object returned by the reducer.

So, here's a utility that handles the typings the way I want in a reducer:

`typescript
interface IMyState {
foo: Number;
}

function myReducer(state: IMyState, action: Action) {
switch(action.type) {
case SET_FOO_ACTION:
// Returns a new object with the type of the first argument.
// Type of the second argument must be a subset of the type of the first argument.

// Fails: bar is not a member of IMyState.
return tassign(state, { foo: 3, bar: 'wat' });
}

return state;
}
``