6. Collections And Data Modeling

Programs need to hold more than one value. Alef gives beginners two simple tools first:

• arrays for ordered lists
• maps for key/value data

When a shape deserves a name, use structs and enums.

Arrays

An array stores values in order.

fn main() {
    let statuses = ["triage", "progress", "review", "done"]
    println(statuses[0])
}

triage

Add an item with push:

fn main() {
    let statuses = ["triage", "progress"]
    statuses.push("done")

    for status in statuses {
        println(status)
    }
}

Maps

A map stores values by key. This is useful for JSON-like data.

fn main() {
    let ticket = {
        "id" => "TD-101",
        "title" => "Checkout timeout",
        "priority" => "P0"
    }

    println(ticket["title"])
}

Checkout timeout

Use maps when:

• data comes from JSON
• an example should be easy to inspect
• keys are dynamic

Structs

Use a struct when a shape deserves a name.

struct Task {
    id: string,
    title: string,
    done: bool
}

fn main() {
    let task = Task {
        id: "task_1",
        title: "Write docs",
        done: false
    }

    println(task.title)
}

Structs make a program easier to read when the same shape appears many times.

Enums

Use an enum when the valid cases are finite.

enum Status {
    Triage,
    Progress,
    Done,
}

Enums are useful for states because they prevent vague strings from spreading through a program.

Choosing The Right Shape

Use this rule of thumb:

• array: many values in order
• map: flexible key/value data
• struct: named fields with a stable shape
• enum: one value from a known set of cases

Exercise

Create a map named customer with:

• name
• plan
• blocked

Write a function that returns "review" when the customer is blocked and "ok" otherwise.