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Counter example part:

// ------ ------
//     Model
// ------ ------

// `Model` describes our app state.
type Model = i32;
Example from a production app (this website)
pub struct Model {
    pub base_url: Url,
    pub page: Page,
    pub selected_seed_version: SeedVersion,
    pub guide_list_visibility: Visibility,
    pub menu_visibility: Visibility,
    pub in_prerendering: bool,
    pub guides: Vec<Guide>,
    pub search_query: String,
    pub matched_guides: Vec<Guide>,
    pub mode: Mode,

  • Model is the state (aka store, data, ..) of your application.

  • In our case it's only a type alias for i32 because we only need to track one value - the number of clicks.

  • It can be almost anything - type alias, enum, etc. However the most Models are structs in real-world apps. And Model has to be static (it basically means that you can't save the most references into it).

How to write a good Model

  • It should be as simple as possible. If there is a way how to derive data from the current Model instead of extending it (i.e. adding more fields), you should derive it, even at the cost of small reduction in performance.

  • Try to save only data into your Model - i.e. add field students: Vec<Student> instead of manager: SchoolManager, where manager contains Vec<Student> and thousand other things. Exceptions are Seed-related items like handles and DOM elements (we will discuss them in other chapters).

  • Don't make your life unnecessary hard.

    • Don't make your Model generic.
    • Don't implement any Model methods (not even default - see detailed explanation at the end of the section).
  • Your Model should be the single source of truth - i.e. add one field selected_menu_item: MenuItemId into your main Model instead of creating many instances of the component MenuItem, where each instance has own Model with field selected: bool.

  • Try to be as expressive as possible and make impossible business rules unrepresentable in code by encoding them with Rust type system.

  • When you need to create custom types that are used in the Model, write them below the Model. (The rule "children below the parent" is valid for all nested structures.) Example:

// ------ ------
//     Model
// ------ ------

struct Model {
  a_field: AType,

// ------ ATYPE ------

enum AType {

Why children below the parent?

Imagine the code with this pattern:

impls for ChildA

You don't know what children are interesting for you because you don't know how and where they are used until you see also the parent.

Human short-term memory can hold only cca 7 items - that means it's very easily overloaded by reading child definitions and as a result the reader will start to jump between children and the parent to empty space and decrease cognitive load.

You can improve DX by moving children below the parent to allow readers to filter interesting children.

Another reason is scanning - readers (especially advanced developers) scan the code and try to recognize familiar patterns or basic building blocks - then blocks like

// ------ ------
//     xxxxx
// ------ ------

xxxxxx Model / init / .. {

effectively work as checkpoints for the eyes.

Why don't implement
Default and other standard traits

Generally all implementations of standard traits (From, Into, Default, Display) are very useful if the item (struct, enum...) is used in multiple contexts or with multiple other items - then the generalization makes sense because it implies you are writing idiomatic Rust and it plays nicely with other standard traits and other items. However when you start to implement standard traits for many items, your code-base is slowly turning into the sea of .into(), ::default(), .to_string(), etc.

As the result:

  • You lose expressive domain-specific names so it can be pretty hard to orient in the code.
  • It bloats the code because standard traits have to cover many cases so they tend to be more complicated.
  • You need to "bend" some parts of your code so it can be written with those implemented traits - it also makes the code harder to read and probably slower.

Default trait: We assume xx::default() calls are pretty cheap operations (see Default for primitive types or Vec) - in the most cases there isn't even memory allocation on the heap and you probably won't find more expensive operations in Default implementations for other items. So when you write more sophisticated Default code for your item and somebody use this item in a nested structure, he will be very surprised once he writes some benchmarks.

One Seed user was even able to accidentally write recursive loop of nested complex Defaults that was causing stack overflow.

In Seed apps, you need to create Model only once, so when you implement Default for Model:

  • It only bloats the code.
  • You'll have tendency to overwrite default values in init function because some Model parts will depend on Url or on other values => worse readability and slower code. (You'll learn about init and Url in other chapters.)
  • It sends misleading signal that Model is/can be created on multiple places.

Single source of truth VS Components

A standard Seed app usually contains one app (aka root or main) Model, several page Models and a few component Models. It's often pretty clear where you should save data and it's simple enough to keep the most of your models in your head during development. So there usually aren't problems with data synchronization or introducing unnecessary models (aka local states).

However it's already possible to integrate Javascript Web Components (we plan to support also Rust Web Components) and React-like Hooks - Seed Hooks - are waiting for integration into Seed. It means it will be pretty easy to create local states (even implicitly when you are using a component library), so there are some tips how to define your Models:

  1. Your business data should be kept in standard Seed Models - ideally in the app model or in page models.

  2. Then you should use state hooks for pure GUI data - e.g. a state variable mouse_over for button component when you want only to switch the button's color on hover.

  3. And you can use Web Components for complex GUI elements or when you often need to interact with JS library / component.

(Don't worry, you'll learn more about Web Components and pages in other chapters; or look at custom_elements and pages examples if you are brave enough.)