Results

Accord operations return tuples. This page covers the helpers for building and consuming them. For the why, see Tuples, not throws.

Import

import { fail, match, ok, trycatch, unexpected } from '@marcelsud/accord'
import type { Result } from '@marcelsud/accord'

Result

type Result<T, E extends Tagged> =
  | readonly [value: T, error: null]
  | readonly [value: unknown, error: E]

Check error !== null to narrow the value.

ok(value)

ok({ balance: 100 }) // [{ balance: 100 }, null]

fail(error)

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

trycatch(task)

Converts thrown or rejected values into Unexpected.

const [data, error] = trycatch(() => JSON.parse(text)) // sync function
const [res, fetchError] = await trycatch(() => fetch(url)) // async function
const [value] = await trycatch(Promise.resolve(3)) // promise

• Prefer passing a function when the work should start later.
• trycatch doesn't flatten an existing tuple. Accord operations already return tuples; don't wrap them.

match(value, cases)

Exhaustive pattern matching on _tag. Handlers receive the narrowed value.

const next = match(event, {
  Opened: () => ({ ...state, open: true }),
  Deposited: ({ amount }) => ({ ...state, balance: state.balance + amount }),
})

Missing or unknown cases fail type checking. Adding a new event or rejection points you at every match that needs updating. At runtime, an unhandled tag throws a TypeError.

unexpected(cause)

Builds an Unexpected error: an Error with _tag: 'Unexpected' and cause.

Built-in error tags

Tag	Shape	When
Unexpected	Error & { _tag, cause }	A throw, or an invalid runtime result
InvalidInput	{ _tag, message }	Input failed a schema
Conflict	{ _tag, expected, actual }	expectedVersion didn't match
CommandConflict	{ _tag, commandId }	A commandId was reused with different input
ConsistencyTimeout	{ _tag, stream, version }	An atLeast read couldn't catch up
Unauthorized, NotFound	{ _tag }	HTTP transport