Function fold

fold<R extends Result.Base, S2, E2>(
    result: R,
    mapFn: (value: UnwrapOk<R>) => S2,
    mapErrFn: (error: UnwrapErr<R>) => E2,
): Result<S2, E2>

Applies one of two functions depending on whether the Result is Ok or Err.

Type Parameters

R extends Result.Base
The input Result.Base type.
S2
The type of the success value returned by mapFn.
E2
The type of the error value returned by mapErrFn.

Parameters

result: R
The Result to fold.
mapFn: (value: UnwrapOk<R>) => S2
The function to apply if result is Ok.
mapErrFn: (error: UnwrapErr<R>) => E2
The function to apply if result is Err.

Returns Result<S2, E2>

A new Result<S2, E2> based on the applied function.

Example

const okValue = Result.ok(2);
const errValue = Result.err('bad');

const foldedOk = Result.fold(
  okValue,
  (value) => value * 2,
  (error) => error,
);
const foldedErr = Result.fold(
  errValue,
  (value: number) => value * 2,
  (error) => error.toUpperCase(),
);

assert.deepStrictEqual(foldedOk, Result.ok(4));
assert.deepStrictEqual(foldedErr, Result.err('BAD'));

const foldNumbers = Result.fold(
  (value: number) => value * 3,
  (error: string) => error.length,
);

assert.deepStrictEqual(foldNumbers(Result.ok(3)), Result.ok(9));
assert.deepStrictEqual(foldNumbers(Result.err('oops')), Result.err(4));

fold<S, E, S2, E2>(
mapFn: (value: S) => S2,
mapErrFn: (error: E) => E2,
): (result: Result<S, E>) => Result<S2, E2>

Applies one of two functions depending on whether the Result is Ok or Err.

Type Parameters

S
E
S2
The type of the success value returned by mapFn.
E2
The type of the error value returned by mapErrFn.

Parameters

mapFn: (value: S) => S2
The function to apply if result is Ok.
mapErrFn: (error: E) => E2
The function to apply if result is Err.

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

A new Result<S2, E2> based on the applied function.

Example

const okValue = Result.ok(2);
const errValue = Result.err('bad');

const foldedOk = Result.fold(
  okValue,
  (value) => value * 2,
  (error) => error,
);
const foldedErr = Result.fold(
  errValue,
  (value: number) => value * 2,
  (error) => error.toUpperCase(),
);

assert.deepStrictEqual(foldedOk, Result.ok(4));
assert.deepStrictEqual(foldedErr, Result.err('BAD'));

const foldNumbers = Result.fold(
  (value: number) => value * 3,
  (error: string) => error.length,
);

assert.deepStrictEqual(foldNumbers(Result.ok(3)), Result.ok(9));
assert.deepStrictEqual(foldNumbers(Result.err('oops')), Result.err(4));

Function fold

Type Parameters

Parameters

Returns Result<S2, E2>

Example

Type Parameters

Parameters

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

Example

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