What are effects?#

In Crochet, Effects are a way of describing the things your program can do—but without worrying about how your program is going to do these things. This might seem a bit silly at first. Why would anyone ever need to describe the things a program can do without describing how the program does them? Isn’t having the program do things precisely why we’re writing one?

And sure enough, if it was just a matter of having the program do things, we could have used commands for that. After all, looking at commands’ signatures tells us what a program does. And looking at commands’ bodies tells us how the program does those things.

So we need some work on motivating the need for all this “effects” business. But keep in mind that the goal of effects is to make writing programs in Crochet both safe and flexible.

Intentions, context, and handlers#

Let’s imagine you’re writing a game that allows players to save the current state of the game, and later on restore it, so players can continue playing from their last checkpoint rather than the very beginning.

To do this, you decide to store the state of the game as a file in the player’s computer. You implement this in your game and things work just fine. Sometime later, you decide to have your game also work in a web browser, so people can play it without having to download anything. “Oh no!”, you realise, “you can’t save files in a web browser!”

Unfortunately, your previous decision of using the player’s file system means that you need to rethink your strategy. This requires significant effort rewriting, but also significant effort to design something that can be used in both contexts.

And what if you wanted to verify that your saving and restoring functions work correctly? How should you approach that? What about using them interactively in the IDE? Or debugging it? Suddenly, not even carefully designing your types and commands to make things consistent across all of these contexts would be enough anymore. You’d need to reach out for more and more complicated concepts.

In order to avoid all of this, Crochet has a principled way of designing these kind of interactions: Intentions, Contexts, and Handlers.


An Intention describes, at a conceptual level, what kind of things your program wants to achieve. In our example we could say we have two intentions:

  • Saving: we want to allow players to ask the game to remember the current state.

  • Loading: we want to allow players to ask the game to go back to a previously remembered state.

In Crochet, these intentions are captured by effect declarations. For this case, we could end up with the following:

effect game-state with
  save(slot is integer);
  load(slot is integer);

We have two cases for this effect: save and load. The parameters described here work similarly to type parameters. So, here we allow players to remember multiple states of the game, by placing each state in a particular slot.

Once we have this effect declared, we can use the perform expression to tell Crochet where we intend for this effects to happen. For example, we could define _ save-at: _ and _ load-from: _ commands that we could then use any time we want the game to be saved or loaded:

command game save-at: Slot =
  perform game-state.save(Slot);

command game load-from: Slot =
  perform game-state.load(Slot);

By themselves, these commands don’t really do anything. They just tell Crochet what our intentions are. But we can move on with building our game by just expressing this first.


Next we have Contexts. A context indicates how we think that fulfilling these intentions would take place. For example, a context for our game-state effect could be the platform that is running the game—running it in a web browser would require us to think about game states in a different way from having the same game run directly on a Windows machine. An android phone would be another different context.

If our intention was having a way of controlling a game’s character, we could think of contexts as different ways in which this character could be controlled: players could use gamepads, keyboards, touch input, voice commands, or even motion capture.

Thinking about more contexts allows us to come up with effect definitions that are more flexible, though that’s not always a goal—remember that the more things you consider, the more work you might have to do to properly support everything.

For this example, let’s consider only “Windows” and “Web Browser” as our context.


Finally, we have Handlers. A handler specifies how effects should be carried out. For example, let’s say that the browser type provides a way of storing and retrieving data in a web browser. We could then implement a handler for our effect in that context as such:

  on game-state.save(Slot) do
    browser at: Slot put: game current-state;
    continue with nothing;

  on game-state.load(Slot) do
    let State = browser at: Slot;
    continue with State;

Here, any part of the program that is executed between the handle and with keywords will have the behaviours we specified in the on handlers for the game-state effect. We similarly need a handler for when the game runs on Windows, and any other context we plan to support.

But how do we choose the right handler based on the context? What is this continue with business? In the next chapter we discuss this and other specifics of handlers.