将联合类型转换为 fp-ts 中的任一类型

Question

In typescript, how can I transform a union type A|B into fp-ts 's Either<A,B> ? It feels natural and there must be a nice way of doing it.

Answer 1

I found that it's impossible.

My original idea was that since union and Either type are both sum type, they are algebraically equal. Therefore there must be a natural and nice way of transforming to each other.

The problem was that at one point you have to typecheck an instance with a generic type but there's simply no way of doing it on Typescript.

Answer 2

Assuming that you have a type number | string number | string you could do the following:

import * as E from 'fp-ts/lib/Either'
import * as F from 'fp-ts/lib/function'

const toEither = F.flow(
    E.fromPredicate(
        x => typeof x === 'string', // Assuming that string is the Right part
        F.identity
    ),
)

Which will produce:

toEither(4) 
{ _tag: 'Left', left: 4 }
toEither('Foo')
{ _tag: 'Right', right: 'Foo' }

BUT Keep in mind that Either is not used to split union types but to wrap the error path and your happy path of your result in one type.

I have only checked the above code through ts-node. I have not seen the actual types generated by TS for the toEither function