Lazily finding first item in an array of TaskEithers

[markgaylard]

Hi. Given the following functions:

declare const getNumber = (id: string): TaskEither<E, O.Option<number>>;

declare const getFirstSuccess = (ids: string[]): TaskEither<E, O.Option<number>>;

I'm trying to work out how to (lazily) find the first O.some result. My initial idea was to implement getFirstSuccess with something like pipe(ids, A.findFirstMap(getNumber)) but that doesn't work because findFirstMap wants a function that returns an Option. I think I can make it work by using A.map(getNumber) then TE.sequenceArray then folding the result and doing a A.findFirst on the result, but ideally I'd like this to only call getNumber until i get the first non-none result.

Is this possible? If so, how? :)

Thanks.

[imcotton]

Could use altAll to perform short-circuit evaluation throughout array:

import {
    alt,
    function as F,
    option as O,
    readonlyArray as A,
    taskEither as TE,
} from 'fp-ts';



declare const getNumber: (id: string) => TE.TaskEither<Error, O.Option<number>>;



const getFirstSuccess: (ids: string[]) => TE.TaskEither<Error, number> = F.flow(

    A.map(F.flow(
        getNumber,
        TE.chain(TE.fromOption(() => new Error('no number'))),
    )),

    alt.altAll (TE.Alt) (TE.left(new Error('empty array'))),

);

Note that in order to play well, you'd want to lift and flat nested Option into Either.