Either

Map Left

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Learn about map left in fp-ts either

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08-map-left.exercise.ts

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Test Results

Requirements

describe('enrichError', () => {
  const userRight: E.Either<ErrorCode, User> = E.right({ id: 1, name: 'Alice' })
  const userLeft1: E.Either<ErrorCode, User> = E.left('ERR_NOT_FOUND')
  const userLeft2: E.Either<ErrorCode, User> = E.left('ERR_INVALID_ID')

  it('preserves right value', () => {
    const result = enrichError(userRight)
    expect(result).toEqual({ _tag: 'Right', right: { id: 1, name: 'Alice' } })
  })

  it('enriches ERR_NOT_FOUND error', () => {
    const result = enrichError(userLeft1)
    expect(result._tag).toEqual('Left')
    if (result._tag === 'Left') {
      expect(result.left.code).toEqual('ERR_NOT_FOUND')
      expect(result.left.message).toEqual('User not found')
      expect(typeof result.left.timestamp).toBe('number')
    }
  })

  it('enriches ERR_INVALID_ID error', () => {
    const result = enrichError(userLeft2)
    expect(result._tag).toEqual('Left')
    if (result._tag === 'Left') {
      expect(result.left.code).toEqual('ERR_INVALID_ID')
      expect(result.left.message).toEqual('Invalid user ID')
      expect(typeof result.left.timestamp).toBe('number')
    }
  })
})
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