catchSome.ts

import { either } from 'fp-ts';
import { pipe } from 'fp-ts/function';
import Either = either.Either;

declare global {
  interface ErrorConstructor<T extends string = string> extends Error<T> {}

  interface Error<T extends string = string> {
    readonly tag: T;
    new<T extends string>(message?: string): Error<T>;
  }
}

export const catchSome = <E extends E1, A, E1 extends Error<string>>(
  handle: ((error: E) => A),
  guard: ((e: E1) => e is E)
) => (ea: Either<E1, A>): Either<Exclude<E1, E>, A> => pipe(
  ea,
  either.match(
    e1 => guard(e1) ? either.right(handle(e1)) : either.left(e1 as Exclude<E1, E>),
    either.right,
  )
);

class A extends Error<'A'> { public readonly tag = 'A'; }
class B extends Error<'B'> { public readonly tag = 'B'; }
class C extends Error<'C'> { public readonly tag = 'C'; }
class D extends Error<'D'> { public readonly tag = 'D'; }

type Errors = A | B | C | D;

declare const eeee: Either<Errors, string>;
const result: Either<B | C | D, string> = pipe(
  eeee,
  catchSome(
    error => error.message,
    (error: Errors): error is A => error instanceof A,
  )
);