# 02 State management

We saw in the qucikstart guide that all Lustre applications are built around the
Model View Update (MVU) architecture. This means that the state of the application
is stored in a single, immutable data structure called the model, and updated as
messages are dispatched to the runtime.

## View functions not components

Although Lustre does have a way to create encapsulated stateful components (something
we sorely missed in Elm) it shouldn't be the default. The word "component" is a bit
overloaded in the frontend world, so for clarify Lustre considers _components_
as stateful nested Model-View-Update applications and calls stateless functions
that return `Element`s _view functions_.

The best Lustre code bases take the lessons learned from similar languages like
Elm, Erlang, and Elixir and keep the number of components low and the number of
simple view functions much higher. If you're coming from a typical frontend
framework the idea of eschewing stateful components might seem quite strange, but
there are some tangible benefits to this approach:

- **Favouring view functions forces us to be intentional with state.**

  ...

- **Components are bad for code organisation.**

  ...

- **Avoiding components makes your code easier to test.**

  When we reach for components too soon or too frequently, we often end up needing
  to pull in a complete E2E testing framework to make sure our code is behaving
  correctly, or we might end up exposing our components' internals for testing:
  defeating the purpose of encapsulation in the first place!

  By sticking to plain view functions and functions to transform data before
  rendering, we end up with a codebase that is much easier to test with Gleam's
  available testing tools.

- **Overusing components makes refactoring more challenging.**

  Imagine you have a table component with tabs to switch between different views.
  If some time in the future you decide to pull the tabs out so they can be
  rendered elsewhere on the page you'll discover that the tabs' state was tightly
  couple to the table. Now we are forced to refactore the table component so the
  tab state can be passed in as an attribute. We'll also need to refactor the
  _parent_ to contain the state of the tabs so it can be passed down to both
  components.

  By avoiding components this sort of refactoring becomes simpler: we were already
  managing the state further up the component tree so moving things around is
  much less painful.

- **Creating components requires boilerplate.**

  Components share the same shape as any other Lustre application. That means for
  any component you want to create, you also need to define an `init`, `update`,
  and `view` function, a `Model` type, and a `Msg` type. If you find yourself
  thinking "wow, this is a lot of boilerplate just to do X" then listen to your
  gut!