Function flatMap

flatMap<R extends UnknownResult, S2, E2>(
result: R,
flatMapFn: (value: UnwrapOk<R>) => Result<S2, E2>,
): Result<S2, E2 | UnwrapErr<R>>

Applies a function that returns a Result to the success value of a Result. If the input is Err, returns the original Err. This is the monadic bind operation for Result.

Type Parameters

R extends UnknownResult
The input UnknownResult type.
S2
The success type of the Result returned by the function.
E2
The error type of the Result returned by the function.

Parameters

result: R
The Result to flat map.
flatMapFn: (value: UnwrapOk<R>) => Result<S2, E2>
The function to apply that returns a Result.

Returns Result<S2, E2 | UnwrapErr<R>>

The result of applying the function, or the original Err.

Example

const parseNumber = (input: string): Result<number, string> => {
  const num = Number.parseInt(input, 10);

  return Number.isNaN(num) ? Result.err('not a number') : Result.ok(num);
};

const parsed = Result.flatMap(Result.ok('42'), parseNumber);

const failure = Result.flatMap(Result.ok('abc'), parseNumber);

const passthrough = Result.flatMap(Result.err('fail'), parseNumber);

assert.deepStrictEqual(parsed, Result.ok(42));

assert.deepStrictEqual(failure, Result.err('not a number'));

assert.deepStrictEqual(passthrough, Result.err('fail'));

const parseThenDouble = Result.flatMap((input: string) =>
  Result.map(parseNumber(input), (value) => value * 2),
);

assert.deepStrictEqual(parseThenDouble(Result.ok('10')), Result.ok(20));

flatMap<S, S2, E2>(
flatMapFn: (value: S) => Result<S2, E2>,
): <E>(result: Result<S, E>) => Result<S2, E2 | E>

Applies a function that returns a Result to the success value of a Result. If the input is Err, returns the original Err. This is the monadic bind operation for Result.

Type Parameters

S
S2
The success type of the Result returned by the function.
E2
The error type of the Result returned by the function.

Parameters

flatMapFn: (value: S) => Result<S2, E2>
The function to apply that returns a Result.

Returns <E>(result: Result<S, E>) => Result<S2, E2 | E>

The result of applying the function, or the original Err.

Example

const parseNumber = (input: string): Result<number, string> => {
  const num = Number.parseInt(input, 10);

  return Number.isNaN(num) ? Result.err('not a number') : Result.ok(num);
};

const parsed = Result.flatMap(Result.ok('42'), parseNumber);

const failure = Result.flatMap(Result.ok('abc'), parseNumber);

const passthrough = Result.flatMap(Result.err('fail'), parseNumber);

assert.deepStrictEqual(parsed, Result.ok(42));

assert.deepStrictEqual(failure, Result.err('not a number'));

assert.deepStrictEqual(passthrough, Result.err('fail'));

const parseThenDouble = Result.flatMap((input: string) =>
  Result.map(parseNumber(input), (value) => value * 2),
);

assert.deepStrictEqual(parseThenDouble(Result.ok('10')), Result.ok(20));

Function flatMap

Type Parameters

Parameters

Returns Result<S2, E2 | UnwrapErr<R>>

Example

Type Parameters

Parameters

Returns <E>(result: Result<S, E>) => Result<S2, E2 | E>

Example

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