Tuples, not throws

Every Accord operation returns a tuple: [value, null] on success, [null, error] on failure. Errors are objects with a _tag.

const [receipt, error] = await bank.account.deposit('a-1', 100)

// receipt is unknown until error is narrowed.
if (error !== null) {
  console.error(error._tag)
} else {
  console.log(receipt.reply.balance)
}

WARNING

Always check the error slot. A successful operation can return null or undefined as its value, so if (receipt) is not a success check.

Three kinds of failure

Tag	Where it comes from
Your rejections, e.g. NotOpen	You returned fail(NotOpen())
InvalidInput	Input failed a schema at a boundary ({ _tag, message })
Unexpected	Something threw, or a handler returned something that isn't a valid tuple

Unexpected is a real Error with _tag: 'Unexpected' and the original thrown value in cause.

TIP

Constructing an event with invalid data throws a TypeError inside your method. The runtime catches it, discards the working instance, and returns Unexpected. If a value can be invalid for business reasons, check it first and return a rejection instead.

Producing results

import { fail, ok, rejection } from '@marcelsud/accord'

const NotOpen = rejection('NotOpen')

ok({ balance: 100 }) // [{ balance: 100 }, null]
fail(NotOpen()) // [null, { _tag: 'NotOpen' }]

Wrap code that throws with trycatch:

import { trycatch } from '@marcelsud/accord'

const [data, error] = trycatch(() => JSON.parse(text))
const [response, fetchError] = await trycatch(fetch(url))

Handling them

Use match for exhaustive handling. Missing or unknown cases fail type checking.

import { match } from '@marcelsud/accord'

const message = match(error, {
  NotOpen: () => 'Open the account first',
  DepositLimit: ({ maximum }) => `The limit is ${maximum}`,
  InvalidInput: ({ message }) => message,
  Unexpected: () => 'Something went wrong',
  Conflict: () => 'Try again',
  CommandConflict: () => 'Duplicate command',
})

See Results for the full API.