Convert Array of Either<E,A> to Either<E,A[]> (sequence function in Scalaz)

To go from Array<Either<L, A>> to Either<L, Array<A>> you can use sequence:

import { array } from 'fp-ts/lib/Array'
import { either } from 'fp-ts/lib/Either'

array.sequence(either)(arrayOfEithers)

Your example can also be further simplified using traverse

array.traverse(either)(['1','2','3'], NumberFromString.decode)