#andThen

Pass an Ok value to another function returning a Result. An Err passes through unchanged, without calling the function. Inspired by Rust's Result::and_then, exposed as a standalone function rather than a method.

#Usage

#andThen(result, next)

import { andThen, err, ok, type Result } from 'massaman/control'

const parseCount = (raw: string): Result<number, TypeError> => {
  const count = Number(raw)
  if (Number.isNaN(count)) {
    return err(new TypeError('Expected a number'))
  }
  return ok(count)
}

const requirePositive = (count: number): Result<number, RangeError> => {
  if (count <= 0) {
    return err(new RangeError('Expected a positive count'))
  }
  return ok(count)
}

const count = andThen(parseCount('42'), requirePositive)
// Result<number, TypeError | RangeError>; Ok(42)

andThen(parseCount('invalid'), requirePositive)
// Original parse failure; requirePositive is not called.

#Parameters

#Returns

Result<U, E | F>. Returns the callback's Result or the original Err by reference. Unlike Rust's method, the input and callback may have different error types; their union is preserved.

#Exceptions and async work

Unexpected callback exceptions propagate; andThen does not catch or normalize them. Use attempt at throwing boundaries. Async callbacks are not accepted: await attemptAsync first, then use andThen for synchronous continuation. For another async step, use an explicit isErr guard and await the operation.

#See also

• orElse — continue on failure instead
• attempt / attemptAsync — capture unsafe work
• Result & Errors