Meiosis Tutorial

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04 - Meiosis with Function Patches

In the previous lesson, 03 - Streams, we started setting up the Meiosis pattern:

Our state had the following shape:

  value: 0

Our patches were numbers such as 1 and -1, and our accumulator applied the patches to the state by adding the number to state.value.

We are going to change our patches and accumulator function to be general-purpose, so that the shape of our state can be much more flexible, and our actions can issue patches to make all sorts of changes to the state.

A Temperature Example

Let's build a temperature example with the following initial state:

  temperature: {
    value: 22,
    units: "C"

We can increase and decrease the value, as well as change the units betwen C (Celsius) and F (Farenheit), converting the value in the process.

We need to:

In this section, we will use one approach using function patches. In the next section, we will look at another approach - my personal favourite - using a small utility called Mergerino.

Using Function Patches

Instead of using plain numbers as patches, which are limited to incrementing a counter, we can use functions. Indeed, we can pass functions onto the update stream and use them in the accumulator to update the state.

These functions receive the current state as a parameter, and return the updated state. For example, to increment the temperature value:

increment: function(amount) {
  update(function(state) {
    state.temperature.value += amount;
    return state;

Now we need to use these function patches in the accumulator function. Remember that the accumulator gets the current state and the incoming patch as parameters, and must return the updated state. Since the incoming patches are functions, we just need to call them:

var states = flyd.scan(function(state, patch) {
  return patch(state);
}, temperature.initial, update);

Using the ES6 arrow function syntax, this would be:

var states = flyd.scan((state, patch) => patch(state),
  temperature.initial, update);

Either way, it's a simple way to create the stream of states.

Putting it all together, we have:


Try it out: notice that the initial state appears in the output on the right. Within the console, type and then press Enter:



In the output on the right, you'll see the updated states.

In the next section, 05 - Meiosis with Mergerino, we will look at an alternative to function patches.

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Meiosis is developed by @foxdonut00 / foxdonut and is released under the MIT license.