True Myth 2.2

Maybe helpers for safe object lookup and Result helpers for exception-throwing code.

October 27, 2018Filed under Tech#functional programming#open source software#true myth#typescriptMarkdown source

I just released v2.21 of True Myth, with two new pairs of helpers to deal with safe JavaScript object property lookup with Maybes and handling exception-throwing code with Results.

Safe JavaScript object property lookup

We often deal with optional properties on JavaScript objects, and by default JavaScript just gives us undefined if a property doesn’t exist on an object and we look it up:

type Person = {
  name?: string;
};

let me: Person = { name: 'Chris' };
console.log(me.name); // Chris

let anonymous: Person = {};
console.log(anonymous.name); // undefined

We can already work around that with Maybe.of, of course:

function printName(p: Person) {
  let name = Maybe.of(p.name);
  console.log(name.unwrapOr('<anonymous>'));
}

But this is a really common pattern! Maybe.property is a convenience method for dealing with this:

function printName(p: Person) {
  let name = Maybe.property('name', p);
  console.log(name.unwrapOr('<anonymous>'));
}

At first blush, this might be a head-scratcher: after all, it’s actually slightly longer than doing it with Maybe.of. However, it ends up showing its convenience when you’re using the curried form in a functional pipeline. For example, if we had a list of people, and wanted to get a list of just the people’s names (ignoring anonymous people), we might do this:

function justNames(people: Person[]): string[] {
  return people
    .map(Maybe.property('name'))
    .filter(Maybe.isJust)
    .map(Just.unwrap);
}

Another common scenario is dealing with the same kind of lookup, but in the context of a Maybe of an object. Prior to 2.2.0, we could do this with a combination of Maybe.of and andThen:

function getName(maybePerson: Maybe<Person>): string {
  return maybePerson.andThen(p => Maybe.of(p.name));
}

This is harder to compose than we might like, and we can’t really write it in a “point free” style, even if that’s more convenient. We also end up repeating the andThen invocation every time we go down a layer if we have a more deeply nested object than this. Accordingly, 2.2.0 also adds another convenience method for dealing with deeply nested lookups on objects in a type-safe way: Maybe.get (and the corresponding instance methods).

// Function version:
function getNameFn(maybePerson: Maybe<Person>): string {
  return Maybe.get('name', maybePerson);
}

// Method version
function getNameM(maybePerson: Maybe<Person>): string {
  return maybePerson.get('name');
}

Again, since the function version is curried, we can use this to create other little helper functions along the way:

const getName = Maybe.get('name');

function getAllNames(people: Maybe<Person>[]): string[] {
  return people
    .map(getName)
    .filter(Maybe.isJust)
    .map(Just.unwrap);
}

And if our object is a deeper type:

type ComplicatedPerson = {
  name?: {
    first?: string;
    last?: string;
  };
};

let none: Maybe<ComplicatedPerson> = Maybe.nothing();
console.log(none.get('name').toString());
// Nothing
console.log(none.get('name').get('first').toString());
// Nothing

let nameless: Maybe<ComplicatedPerson> = Maybe.just({});
console.log(nameless.get('name').toString());
// Just([object Object]);
console.log(nameless.get('name').get('first').toString());
// Nothing

let firstOnly: Maybe<ComplicatedPerson> = Maybe.just({
  name: {
    first: 'Chris',
  },
});
console.log(firstOnly.get('name').toString());
// Just([object Object]);
console.log(firstOnly.get('name').get('first').toString());
// Just(Chris);

Note that in these cases, since the type we’re dealing with is some kind of object with specific keys, if you try to pass in a key which doesn’t existing on the relevant object type, you’ll get a type error. (Or, if you’re using the curried version, if you try to pass an object which doesn’t have that key, you’ll get a type error.) However, we also often use JavaScript objects as dictionaries, mapping from a key to a value (most often, but not always, a string key to a specific value type). Maybe.property and Maybe.get both work with dictionary types as well.

type Dict<T> = { [key: string]: T };

let ages: Dict<number> = {
  'chris': 31,
};

console.log(Maybe.property('chris', ages)); // Just(31)
console.log(Maybe.property('joe', ages)); // Nothing

let maybeAges: Maybe<Dict<number>> = Maybe.of(ages);
console.log(ages.get('chris')); // Just(31)
console.log(ages.get('joe')); // Nothing

Hopefully you’ll find these helpful! I ran into the motivating concerns for them pretty regularly in the codebase I work with each day, so I’m looking forward to integrating them into that app!

Handling exception-throwing functions

The other big additions are the Result.tryOr and Result.tryOrElse functions. Both of these help us deal with functions which throw exceptions. Since JavaScript doesn’t have any native construct like Result, idiomatic JavaScript does often throw exceptions. And that can be frustrating you want to have a value type like a Result to deal with instead.

Sometimes, you don’t care what the exception was; you just want a default value (or a value constructed from the local state of your program, but either way just one value) you can use as the error to keep moving along through your program. In that case, you wrap a function which throws an error in Result.tryOr. Let’s assume we have a function either returns a number of throws an error, which we’ll just call badFunction because the details here don’t really matter.

const err = 'whoops! something went wrong!';
const result = Result.tryOr(err, badFunction());

The result value has the type Result<number, string>. If badFunction through an error, we have an Err with the value 'whoops! something went wrong!' in it. If it didn’t throw an error, we have an Ok with the number returned from badFunction in it. Handy!

Of course, we often want to do something with the exception that gets thrown. For example, we might want to log an error to a bug-tracking service, or display a nice message to the user, or any number of other things. In that case, we can use the Result.tryOrElse function. Let’s imagine we have a function throwsHelpfulErrors which returns a number or does just what it says on the tin: it throws a bunch of different kinds of errors, which are helpfully distinct and carry around useful information with them. Note that the type of the error-handling callback we pass in is (error: unknown) => E, because JS functions can throw anything as their error.

const handleErr = (e: unknown): string => {
  if (e instanceof Error) {
    return e.message;
  } else if (typeof e === 'string') {
    return e;
  } else if (typeof e === 'number') {
    return `Status code: ${e}`;
  } else {
    return 'Unknown error';
  }
}

const result = Result.tryOrElse(handleErr, throwsHelpfulErrors);

Here, result is once again a Result<number, string>, but the error side has whatever explanatory information the exception provided to us, plus some massaging we did ourselves. This is particularly handy for converting exceptions to Results when you have a library which uses exceptions extensively, but in a carefully structured way. (You could, in fact, just use an identity function to return whatever error the library throws—as long as you write your types carefully and accurately as a union of those error types for the E type parameter! However, doing that would require you to explicitly opt into the use of any to write it as a simple identity function, so I’m not sure I’d recommend it. If you go down that path, do it with care.)


And that’s it for True Myth 2.2! Enjoy, and of course please open an issue if you run into any bugs!

Thanks to Ben Makuh for implementing Result.tryOr and Result.tryOrElse. Thanks to Ben and also Frank Tan for helpful input on the Maybe.get and Maybe.property API design!


  1. I published both 2.2.0 and 2.2.1, because once again I missed something along the way. This time it was making sure all the new functions were optionally curried to support partial application.