9 min read

Behaviors and streams, why both?


Functional reactive programming (FRP) has historically included two different abstractions over time: behavior and stream. Today most FRP and FRP-inspired libraries only have a single abstraction over time. This one abstraction is typically called “stream” or “observable”.

People used to these libraries may wonder: Why do I need both behavior and stream? I’m doing fine with just streams/observables. Asking that is natural. In fact, when I wrote my first FRP library Flyd I only included a single abstraction over time. I thought it was simpler than having two concepts.

However, after digging deeper into FRP I came to see that one loses something very crucial when not making a distinction between behaviors and stream. What exactly that is is not obvious, however. In this blog post, I will try to explain what it is.

What is the difference anyway?

Both behaviors and streams represent things that happen or changes over time. But still, they are very different. Visually this difference looks like this.

Diagram of behavior and stream

Intuitively, a behavior is a value that changes over time. And a stream is something that has occurrences at specific moments in time. A behavior can be seen as a function over time. A stream, on the other hand, is a list of events associated with their time of occurrence.

To figure out whether something is conceptually a behavior or a stream one can simply ask: Does this thing has a “current value” or does it instead have a “last occurrence”? In the first case, it is a behavior and in the later case a stream.

The classic example is the mouse. Its position is a behavior while the clicks of its buttons are streams. Here are a few additional examples:

  • Sunset is a stream. It doesn’t have a current value but it does have a last occurrence.
  • The position of the sun is a behavior since it always has a current value.
  • The height of a tree is a behavior since it has a current value.
  • Leaves falling off of a tree is a stream—we can tell when a leave last fell off the tree.

As you can see things in the real world are either a behavior or a stream. So it seems natural that our programs should be able to express the difference as well.

How can one get away without both?

Most libraries that only have a single abstraction over time has one that is much more like a stream than like a behavior. They pretty much just lack behavior altogether. Whenever people say things such as “an observable is like a list over time” they are talking about streams. They can’t be talking about a behavior because a behavior is a function over time.

How do they compensate for the lack of behaviors? Well, essentially one just “interprets” a stream as a behavior. The image below illustrates this.

Stream as behavior

On the left, we see an actual behavior and on the left we see a stream interpreted as a behavior. We simply remember the last occurrence and takes it to be the “current value” of the stream. Some libraries remember event occurrences like this by default while other have a variant of their stream/observable that does. So even though some libraries doesn’t recognize behaviors as a separate thing they still need features to fill in the gap.

The crucial question now is: What are the downsides to only supporting this “fake” behavior? What are the benefits of having an actual behavior separate from streams? Hang tight, because that is what the rest of this blog posts covers.

It is precise and explicit

When programming it is generally good practice to use types that are as precise as possible. For instance, even though we could, in theory, represent all numbers as strings, we don’t. Even when programming in a dynamic language like JavaScript it is a good idea to have an idea about which types of values your variables contain. For instance, you may be thinking things like “this variable is a number” and “this variable is a string”.

Likewise, when programming with FRP it is beneficial to know which things in a program are behaviors and which things in a program are streams. Conceptually these are two different things! Asking yourself whether a certain phenomenon is a behavior or a stream is highly useful. It is a useful mental process that makes you more aware of what exactly you are dealing with. Making a distinction between behavior and streams gives us richer vocabulary. If your program also makes the distinction this richness will translate into programs that are more expressive and precise about they are talking about.

On the other hand, expressing both behaviors and streams with a single abstraction makes it is impossible to make a clear distinction. That can create confusion and inhibit features and performance because two separate concerns are mixed into a single abstraction.

It prevents mistakes

Libraries that makes a distinction between behaviors and streams can prevent many errors from happening.

For instance, a behavior always has a current value. But, streams doesn’t. This means that when a stream is used as a behavior the user will have to remember to supply some initial occurrence to the stream. If the user forgets that, a bug has been introduced.

A library that recognizes behaviors can know exactly when it is dealing with such. When a current value is expected the API will require a behavior. And the API makes it impossible to create behaviors that don’t have an initial value. This completely eliminates errors where initial values are missing.

Another thing we can prevent with explicit behaviors is meaningless operations. There are a bunch of operations that we can apply to a stream that does not make sense on a behavior. Likewise, there are operations that make sense on a behavior but not on a stream. Libraries that can’t tell the difference between behaviors and streams can’t prevent people from carrying out operations that don’t make sense.

As an example, it makes sense to combine two streams by merging their occurrences. Visually it looks like this

Diagram of combining streams

However, combining two behaviors in this manner doesn’t make any sense. But still, libraries that don’t support behaviors can’t prevent it. We may end up merging the position of one object with the position of another object. This will a result that no longer makes sense understood as a behavior. This can lead to code that does “shady” or confusing things by exploiting that behaviors are represented as stream.

It is a higher abstraction

Some behaviors have multiple representations as a streams. For instance, these two streams represent the exact same behavior.

Diagram of combining streams

Clearly, these “extra” occurrences don’t change the streams meaning as a behavior. But a stream-only library doesn’t know that. Since the library doesn’t have the behavior abstraction but uses streams instead its API necessarily exposes how many occurrences a steam is made of. Even when it’s used as a behavior.

When implementing a behavior, the API can be designed such that “peeking into” the internal representation of a behavior in this manner is impossible. The library knows when the user actually wants to use a behavior. This allows for more optimizations since no implementation details are exposed.

For instance, when implementing behavior with a dependency graph only actual changes in values have to be propagated. As an example of this, let’s say a user writes the following:

const flooredNumberBehavior = numberBehavior.map(Math.floor);

Clearly some changes to numberBehavior won’t lead to a change in flooredNumberBehavior. Maybe an update flows down that changes numberBehavior from 3.5 to 3.8. In this case an implementation of behavior can recognize that nothing changes in flooredNumberBehavior. This allows it to simply stop propagating changes down.

This can lead to substantial improvements to performance as it prevents needles re-computation. To , such an optimization can never be done. Because someone might call a method like scan later that exposes the number of occurrences and not just changes in value.

It allows for infinite resolution behaviors

While the stream interpreted as behavior, we saw above, can represent some behaviors it can’t represent all behaviors. Some behaviors it can only crudely approximate. That is because a behavior can change “infinitely often”. I.e. be continuous. An example of such a behavior is seen below.

Approximating behavior with stream

To the right, we have an attempt at approximating the behavior with a stream. Clearly, such an approximation is lossy and imprecise.

Being able to represent these types of behaviors with infinite resolution is extremely beneficial. In particular, it is helpful when writing programs that deal with things such as time and motion. Like when implementing animations for instance.

One may think that this problem can be avoided simply by using streams with a resolution that is “good enough”. But the problem is not only related to resolution. It is also a problem about composition.

For instance, let’s say you want to implement an animation with streams. You may think, I’ll just create a stream that has an occurrence for each frame. Clearly, that resolution is good enough. But consider what then happens if you combine two such streams. For instance, you may have an x-coordinate that changes every frame and a y-coordinate that does the same. If you combine those you get a stream that changes twice every frame. That is pretty bad.

With an FRP library that support continuous behaviors that problem does not exist. You’d simply have a behavior for the x-coordinate and one for the y-coordinate. Internally those will be represented in a way that supports infinite resolution. Thus, when you combine them you simply get a third behavior, also of infinite resolution.

The above problem is similar to vector graphics vs. pixel graphics. Vector graphics has infinite resolution. This means that we can, among other things, zoom in and rotate without losing quality. Of course, at some point, a vector image will have to be converted to pixels in order to be displayed on the screen. But such a conversion only happens at the very last step, after we have zoomed and rotated. Similarly, when working with behaviors of infinite resolution all operations on them happens to the internally infinite version. Only for the purpose of showing them on the screen are they converted to a format with the necessary resolution.


We have seen some of the main benefits of recognizing that behaviors and streams are two different things. It may up front seem like having two abstractions is more complex that just one. But it turns out that down the road it makes things much simpler and more powerful. In general, programs that keep separate things separate are easier to understand.