🔀 Create a guide on using and creating side effects. (#93)

* :memo: Explain why managed effects are useful.

* :memo: Write a short explainer on pure functions.

* :memo: Finish side effects guide.

* :wrench: Add side effects guide to generated pages.

1 files changed, 55 insertions, 0 deletions

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+# Pure functions
+
+Throughout Lustre's documentation you may come across references to _purity_ or
+_pure functions_. Lustre makes some fundamental assumptions around purity in your
+programs so it's important to know what it means!
+
+## Functions as formulas
+
+Outside of programming, the concept of a "function" exists in mathematics too.
+There, they are sometimes referred to as _formulas_ and you might have already
+encountered them in school, something like:
+
+```
+f(x) = 3x
+```
+
+Or "a function over `x` is equal to `3` times `x`". Let's jump back to Gleam and
+write that function again:
+
+```gleam
+fn f(x) {
+  3 * x
+}
+```
+
+Functions like this are _pure_. They take an input, perform some computation, and
+return an output. The output is _only_ determined by the input, and the function
+doesn't change anything about the outside world like writing to a file or making
+a HTTP request.
+
+**Lustre assumes your `init`, `update`, and `view` functions are pure** and breaking
+this assumption can lead to unexpected behaviour. It is _really_ important to
+make sure these functions are pure!
+
+## Functions as procedures
+
+Of course, what sets programming apart from mathematics is that we _can_ have
+side effects in our programs. You may sometimes see functions that perform side
+effects referred to as _procedures_, and they are useful too!
+
+Lustre may expect your `init`, `update`, and `view` functions to be pure, but
+that doesn't mean _Gleam_ does. To learn more about how Lustre handles side
+effects and procedures, check out the [side effects guide](https://hexdocs.pm/lustre/guide/03-side-effects.html).
+
+## Other resources
+
+Here are some other resources from around the Web that you might also find useful:
+
+- [Keeping components pure](https://react.dev/learn/keeping-components-pure) from
+  the React docs.
+
+- [Pure functions](https://elmprogramming.com/pure-functions.html) from the
+  "Beginning Elm" book.
+
+- [Pure functions](https://en.wikipedia.org/wiki/Pure_function) from Wikipedia.