Result

folktale.result

• Result is like an Either, modeling either a value or an error.
• It is a Monad as it has a chain property.
• It will be either a successful value: Result.OK, or an error: Result.error.
• The value will be stored in the property value, whether Ok or an error.
• Will have either an isOK or isError property, but these are deprecated.

Creation

Result.Ok(value);
Result.Error(value);

Extracting Result

getOrElse

It also offers a getOrElse method which can be used for extracting a value with a default if the result is an error:

const value = result.getOrElse(DEFAULT_VALUE);

Transformation

matchWith

To map the result to a different function depending on whether it is a Result.Ok or a Result.Error:

result.matchWith({
  Ok: ({value}) => `Value: ${value}`,
  Error: () => `No value`,
})

map

To perform an action on the result only if it is OK, map can be used. If the result is a Result.Error, it will just return that Result.Error, but if the result is Result.Ok, its value will be passed in to the function supplied, and it will return whatever the function returns as the value of a Result.Success.

const value = result.map(example);

chain

However, if the function supplied to map also returns a Result, chain should be used instead to avoid a nested Result. In this case it is assumed that the value returned will be a Result, so whatever is returned from the function is returned as is, without being wrapped in a Result.

const value = result.chain(example);

orElse

To perform an action on the result only if it is an Error, orElse_ can be used. _If the result is a Result.Ok, it will just return that Result.Ok, but if the result is Result.Error, its value will be passed in to the function supplied, and it will return whatever the function returns. For consistency it should return a Result, unless it throws.

const value = result.orElse(example);

Introspection

To devine whether a Result is an Ok or an Error , hasInstance is used:

Result.Ok.hasInstance(result);
Result.Error.hasInstance(result);

Examples

Basic Chaining

const gt10 = gt(__, 10);

const alpha = (n) => {
  const result = isNumber(n) ?
    Result.Ok(n) :
    Result.Error(`Value wasn't a number`);
  return result;
}

const beta = (n) => {
  const result = gt10(n) ?
    Result.Ok(n) :
    Result.Error(`Value wasn't > 10`);
  return result;
}

const successfulResult = alpha(22).chain(beta);
const failingFirstResult = alpha('a').chain(beta);
const failingSecondResult = alpha(6).chain(beta);

console.log('Success', successfulResult.value); // 22
console.log('Success - was OK', Result.Ok.hasInstance(successfulResult)); // true
console.log('Success - was Error', Result.Error.hasInstance(successfulResult)); // false

console.log('failedFirst', failingFirstResult.value); // Value wasn't a number
console.log('failedFirst - was OK', Result.Ok.hasInstance(failingFirstResult)); // false
console.log('failedFirst - was Error', Result.Error.hasInstance(failingFirstResult)); // true

console.log('failedSecond', failingSecondResult.value); // value wan't > 10
console.log('failedSecond - was OK', Result.Ok.hasInstance(failingSecondResult)); // false
console.log('failedSecond - was Error', Result.Error.hasInstance(failingSecondResult)); // true

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