interface ShowMessage {
  showDecoder: (a: t.Decoder<any, any>) => O.Option<string>
  showKey: (a: string) => O.Option<string>
  showValue: (a: unknown) => O.Option<string>
}

export const show = (as: t.Errors, showMessage?: Partial<ShowMessage>): string => ...

export const withDefault = <T extends t.Mixed>(
    type: T, defaultValue: t.OutputOf<T>)
    : t.Type<t.TypeOf<T>, t.OutputOf<T>> => ...

import * as t from 'macoolka-io'
import { right } from 'fp-ts/lib/Either'

const M = t.withDefault(t.string, '123')
expect(M.decode(null)).toEqual(right('123'))
expect(M.decode(null)).toEqual(right('123'))
expect(M.decode('4')).toEqual(right('4'))

const MA = t.type({
  name: t.string,
  names: t.array(t.string)
})

const MB = t.type({
  name: t.withDefault(t.string, '1'),
  names: t.withDefault(t.array(t.string), [])
})
expect(MB.decode({})).toEqual(right({ name: '1', names: [] }))
expect(MB.decode({ name: '2' })).toEqual(right({ name: '2', names: [] }))
expect(MB.decode({ names: ['1'] })).toEqual(right({ name: '1', names: ['1'] }))
expect(MB.decode({ names: ['3'], name: '3' })).toEqual(right({ name: '3', names: ['3'] }))
expect(isLeft(MA.decode({}))).toEqual(true)