11 min read

Introducing Jabz


Jabz is a new library aimed at implementing powerful abstractions in JavaScript while being as practical as possible. It is hugely inspired by Fantasy Land but is an attempt at achieving something more convenient and with better performance.

Jabz specifies the same abstractions as Fantasy Land. Functor, monad, traversable, etc. Besides specifying these things it is also a library that includes a common set of functions for working with the abstractions, as well as often used implementations.

I initially created Jabz to be a TypeScript implementation of common structures satisfying the Fantasy Land specification, but along the way I had some ideas that warranted a new specification. In this blog post I will explain how Jabz’s specification of these abstractions differ from Fantasy Land’s. I will assume that the reader is familiar with Fantasy Land and the related abstractions.

Jabz can be found on GitHub.


My overlaying goal was to create a specification and a library that achieved the following properties:

  • Convenience. Using the abstractions should be as convenient as possible from an end users perspective. Developer ergonomics should be highly valued.
  • Performance. Inefficient abstractions are useless abstractions. The specification should give implementations the necessary room for creating performant implementation.
  • Power. The abstractions should be as powerful as can be. That is, they should have as many features and support as many use cases as possible.

To achieve the above, the one thing I traded away was simplicity of specification. Fantasy Land is a simple and elegant specification that only concerns itself with what is strictly essential. Jabz, in comparison, is more voluminous. I do, however, believe the trade-off is worth it.

Below I will describe the major general differences between Jabz and Fantasy Land. After that I will cover some of the specific abstractions and how they are different in Jabz.

Non-prefixed method names

Jabz uses non-prefixed method names. To be a functor an object must have a map method. This is in contrast to Fantasy Land, where a Functor must have a method named fantasy-land/map. Jabz uses non-prefixed method names primarily since they’re more convenient and convenience is one of the primary goals.

One argument for prefixed method names in Fantasy Land is that then libraries can implement the specification even if they currently have methods with the same name that don’t behave according to the specification. In my opinion, disallowing that is a good thing. Having a map method that does one thing and a fantasy-land/map method that does another thing is a source of confusion. It creates situations where map(f,foo) might do something different than foo.map(f).

Requiring non-prefixed methods ensures that if a structure supports Jabz it is not allowed to have, for instance, an improper map method. Jabz demands more from implementations. But it also means that users can rely on implementations having easily accessible methods that behave as expected.

Beyond minimal complete definitions

Each abstraction specified by Fantasy Land is defined as a set of methods. For instance, foldable is defined by a reduce method (I prefer the name foldr so I’ll use that going forward).

Part of the reason why the foldable abstraction is useful is that, building on top of this single method, we can create many more. For instance, we can derive a function for getting the number of elements in any foldable.

function size(foldable) {
  return foldable.foldr((n, m) => n + m, 0);

This is an example of a derived method.

A derived method is one that can be defined in terms of other, often more fundamental, methods.

In all cases the number of methods that Fantasy Land requires by its implementations are as few as possible. What it defines constitutes a minimal complete definition of the given abstraction.

A minimal complete definition is a set of methods describing an abstraction where none of the methods can be derived in terms of the other.

However the size function derived above is problematic. It takes O(n) time, where n is the size of the foldable. Most data structures maintains a size that can be obtained in constant time. Thus, for these data structures using the generalized size function incurs a prohibitively expensive overhead. In practice this means that our abstracted size function, sadly, isn’t all too useful. But with the Fantasy Land specification we can’t do better.

An abstraction that is unnecessarily costly with regards to performance is often impractical.

Haskell solves this issue by including a size method as part of the Foldable type class. Jabz takes a similar approach by specifying that foldables must have a size method. This means that implementations of foldable can optionally implement a performant version of size. Alternatively, they can choose to rely on the default, slower one that Jabz provides.

This is a general trend: where Fantasy Land only contains minimal complete definitions in the specification, Jabz on the contrary includes any method that some specific implementations might benefit from implementing in a specialized way.

Supported by code

The fact that Jabz specifies a lot more methods than Fantasy Land places an extra burden on implementations. Therefore these are meant to be created with support from the library. This makes it very convenient to implement the abstractions.

Jabz achieves this by offering functions that take classes and ensures that they have the necessary methods. These functions only require that some minimal complete definition is present. Beyond that, missing methods will automatically be filled in with default derived implementations.

For instance, Jabz requires functors to have both a map and a mapTo method. But since mapTo can be derived from map, an implementation doesn’t have to specify it. A functor can be implemented like this

class MyFunctor {
  map(f) {

The functor function ensures that MyFunctor satisfies the functor specification. It automatically installs a default derived mapTo on MyFunctors prototype. Each abstraction in Jabz comes with a matching function for creating implementations. Using them as a decorators, as above, looks nice but is of course not required. Decorators are just fancy syntax for applying functions to classes.

Monoid with nicely named methods

Fantasy Land monoids must have the two methods empty and concat. These names make sense for the list instance of monoid where empty gives the empty list and concat does list concatenation. However, for other instances they are very awkward. One example is the Max monoid whose elements are numbers and where infinity is the identity element and the merge operation returns the minimum of two numbers. In this case empty is a very counterintuitive name for a function that returns infinity.

In Jabz the monoid methods are instead called identity and combine. These names should be fairly easy to understand while ensuring that no specific instances of Monoid ends up with non-intuitive names. The idea is that abstract concepts should have abstract names.

Speedy applicatives

If we have a function that takes one argument, we can apply it to a functor with map. However, we can’t apply a function that takes n arguments to n functors. That is what applicative was made for.

If f is a function from three arguments and a, b and c are applicatives, we can use Fantasy Lands ap like this to apply f to the applicatives:


This an awkward way of applying a function to two applicatives. So we’d probably abstract the pattern into a function called lift. It might work like this:

lift(f, a, b, c);

However, there is a problem here. Let’s say f is any function taking n arguments. If we only have the methods ap and of, there is no way to apply f to n applicatives without currying f and calling the curried function n times. This hurts performance pretty badly. In most cases it may not matter. But why have applicatives with an unnecessary performance overhead?

Unplaced with the situation, Jabz allows applicatives to bring their own lift method. lift can be derived, so if they don’t they will be given the default slow one. This has the significant advantage that for a specific structure it is often easy to implement lift so that the function to lift does not have to be curried and is only applied once.

To see what the difference can be in practice I’ve created a small benchmark that compares lifting a function over Jabz’s Maybe with lift and with ap. The source code can be found here.

-------------------- `ap` vs `lift` on `Maybe` --------------------
lift                        9150491.49 op/s ±  4.45%   (68 samples)
ap                          1665864.27 op/s ±  1.31%   (85 samples)
---------------------------- Best: lift ----------------------------

Of course this is a small micro-benchmark but it does show that lifting a function with ap is expensive. Since lifting is pretty much what applicatives are good for, this means that Jabz applicatives come with no overhead, whereas Fantasy Land’s come with an inherent slowdown.

Monads with do-notation

When Haskell began using monads, do-notation was introduced along with them. The reason is simple: monads are not very convenient to work with without do-notation. Hence, I believe that in order to make monads practically useful in JavaScript, we need a substitute for do-notation. Fortunately, such a thing is possible by using JavaScript generators. I first saw this brilliant idea in the library Fantasy Do. Jabz implements this and calls it “go-notation”—because do is a reserved word in JavaScript. It might stand for “Generato-dO-notation”. With this technique we can get do-notation like this.

const value = go(function*() {
  const a = yield someFunctionReturningAMonad(1);
  const b = yield iAlsoReturnAMonad(2);
  return doSomethingWithTheBoundValues(a, b);

which is comparable to

value = do
  a <- someFunctionReturningAMonad(1)
  b <- iAlsoReturnAMonad(2)
  return doSomethingWithTheBoundValues(a, b)

So the string “= yield” should be read as Haskell’s “<-”.

This form of do-notation is immensely useful. But due to some unfortunate technical limitations of generators, the do function has to behave very differently for monads that invoke the callback to chain several times and those that only invoke it once. Therefore, Fantasy Do exports two different functions for these two cases. This seems like a minor inconvenience, but it breaks the abstraction. A single application of do-notation can no longer work for all monads as it should.

Let’s say we have an interface for monads with a random method. The random method takes two integers and returns, in the monad, a random integer between the two. Then we might write code like this:

function veryRandom(m, n) {
  return do(function*() {
    const a = yield m.random(0, n);
    const b = yield m.random(n, n * n);
    return m.of(a + b);

The example is fairly silly, but the point is that veryRandom should work for any monad m that has a random method. One monad that might implement this is IO, which would actually generate a random variable. Another candidate is the non-determinism monad, aka. the list monad which would instead return a list of values in the given range. But since IO only calls its chain parameter once, and List calls it several times, Fantasy Do would require us to use two different types of do-notation.

The end result is that code which should work for all monads ends up only working for some monads. This breaks the abstraction. To fix this, Jabz simply requires all monads to have a multi property. The property declares whether or not the monad invokes the callback to chain multiple times. This small addition to the specification makes completely general do-notation possible, which in turn makes monads a lot more useful.


Foldables in Jabz benefits significantly from the inclusion of more methods in the specification. But I’ve talked about that already. So let’s instead look at how Jabz regains some power that was otherwise lost to the fact that JavaScript isn’t lazy.

Consider the aforementioned find.

function find(predicate, foldable) {
  return foldable.foldr((e, a) => predicate(e) ? just(e) : a, nothing());

Had we coded this nifty function in Haskell, execution would have stopped as soon as an element in the foldable satisfying the predicate had been found. So is the beauty of laziness. JavaScript, on the other hand, is a bit of a workaholic, so the actual function above would always iterate over the entire foldable. In many cases, that will be a lot of wasted effort.

Jabz remedies the situation by adding shortFoldr and shortFoldl to its foldable specification. They are like the normal folds except that the accumulator function has to return a value wrapped in an Either. right means “keep going” and left means “pull the breaks, I’m done”. Both of these are deriveable, but implementations will have to implement them to get the benefits of short-circuiting.

This makes it feasible to implement quite a bunch of additional derived functions compared to what only a strict right fold gives us. Examples are find, findLast, take and any.

Additionally, this also means that infinite data structures can be used with Jabz’s foldable. In fact, Jabz ships with a simple infinite lazy list.

take(5, map((n) => n * n), naturals); //=> [0, 1, 4, 9, 16]

Here naturals is an infinite list of the natural numbers. First we square them with map and then we take the first five with take. This is possible since take utilizes the shortFoldl method on foldables.


We’ve covered some of the main differences between Jabz and Fantasy Land. I hope I’ve convinced you that Jabz brings some interesting ideas to the table. Especially in terms of creating low-overhead abstractions with as many features as one can squeeze out of the them.

Jabz is still far from finished. As a specification Fantasy Land is more comprehensive. Jabz currently only specifies functor, applicative, monad, foldable and traversable. And while it does provide both a healthy set of utility function for working with the abstractions and some commonly used implementations of the specification there is still a lot to add. Contributions and feedback is much appreciated.

The library can be found on Gitub.