export interface EitherC<L extends t.Mixed, R extends t.Mixed>
  extends t.Type<Either<t.TypeOf<L>, t.TypeOf<R>>, EitherOutput<t.OutputOf<L>, t.OutputOf<R>>, unknown> {}

export type EitherOutput<L, R> = LeftOutput<L> | RightOutput<R>

export type LeftOutput<L> = { _tag: 'Left'; left: L }

export type RightOutput<R> = { _tag: 'Right'; right: R }

export function either<L extends t.Mixed, R extends t.Mixed>(
  leftCodec: L,
  rightCodec: R,
  name: string = `Either<${leftCodec.name}, ${rightCodec.name}>`
): EitherC<L, R> { ... }

import { either } from 'io-ts-types/lib/either'
import { left, right } from 'fp-ts/lib/Either'
import * as t from 'io-ts'
import { PathReporter } from 'io-ts/lib/PathReporter'

const T = either(t.string, t.number)

assert.deepStrictEqual(T.decode(right(1)), right(right(1)))
assert.deepStrictEqual(T.decode(left('a')), right(left('a')))
assert.deepStrictEqual(PathReporter.report(T.decode(right('a'))), [
  'Invalid value "a" supplied to : Either<string, number>/1: Right<string>/right: number'
])