Fp-ts: Use Symbol.iterator

[bioball]

ES6 has support for custom iterators.

It'd be great if we could implement Symbol.iterator, so we could write code like so:

const foo: Option<string> = getFooSomeHow();
const bar: Option<string> = getBarSomeHow();

const fooBar = [...foo, ...bar];

[joshburgess]

@bioball What would happen in the case of foo or bar being None in the above example? Would the spread operator simply do nothing?

[bioball]

In that case, the spread operator expands it to nothing, and fooBar would end up being an empty array.

[joshburgess]

Hmmm, I think it's an interesting proposal, but it's also sort of outside of the norm of how most languages use the Maybe/Option type...

You could also write something like this and use it as a wrapper for such cases:

export const toSpreadable = <A>(opt: Option<A>): A[] => opt.fold([] as A[], x => [x])

Like:

const fooBar = [...toSpreadable(foo), ...toSpreadable(bar)]

[gcanti]

Note: toSpreadable is a natural transformation and can be derived from the Foldable instance

import { toArray } from 'fp-ts/lib/Foldable2v'
import { option } from 'fp-ts/lib/Option'

const toSpreadable = toArray(option)