jesper - 4 months ago 25

Javascript Question

I'm trying to get familiar with functional programming in JavaScript. I've just read that pointer functor is:

An object with anfunction that puts any single value into it.`of`

ES2015 addsmaking arrays a pointed functor.`Array.of`

And my question is what does exactly mean "single value"?

I want to make a Functor/Container (like in https://drboolean.gitbooks.io/mostly-adequate-guide/content/ch8.html) that holds grid of given dimension (width, height) as 1-dimensional array and allows me to do transformations on it. As a plain object I would store it as

`{ width: 2, height: 2, list: [1, 2, 3, 4] }`

I know that it's perfectly fine to use pointed functor like this to store single value:

`Container.of(47)`

But is it ok to use object as value assuming object is a "single value":

`Grid.of({ width: 2, height: 2, list: [1, 2, 3, 4] })`

Or even like this:

`Grid.of(2, 2, [1, 2, 3, 4])`

Answer

But is it ok to use object as value assuming object is a "single value":

Yes. `of`

is supposed to take *any* value and put it inside the container. An object certainly is such a single value.

`Grid.of(2, 2, [1, 2, 3, 4])`

No. `of`

is supposed to take a single parameter. If you want to put multiple values inside a functor, put them inside an other structure before and put that structure inside the functor, or construct the functor by something else than its point function (`of`

).

`Grid.of({ width: 2, height: 2, list: [1, 2, 3, 4] })`

No, if you expect that to return the input then it won't work. `of`

should take the input as-is and wrap the structure around it. In case of your grid, it would most certainly look like this:

```
// Grid<A>
class Grid {
// Int -> Int -> [A] -> Grid<A>
constructor(w, h, vals) {
assert(Number.isInteger(w) && Number.isInteger(h));
this.width = w;
this.height = h;
const list = Array.from(vals);
assert(list.length == w * h);
this.list = list;
}
// Grid<A> -> (A -> B) -> Grid<B>
map(f) {
return new Grid(this.width, this.height, this.list.map(f));
}
// A -> Grid<A>
static of(x) {
return new Grid(1, 1, [x]);
}
}
```

So the above call would create a `Grid`

of objects, not a grid of four numbers. Notice that `of`

is not the only way to construct an instance of a functor, it's only the way to construct an instance from a single element.

Notice that `of`

is most important as part of an Applicative, not so much interesting for ordinary Functors. Btw, if you're interested in functional programming concepts, you should also be able to make your `Grid`

a Monoid, a Traversable and a Monad - see https://github.com/fantasyland/fantasy-land.