2. Give the data a shape

Right now each event is a bare string. Before parsing it, give it named fields — without declaring a type.

Anonymous records

A record is a set of labelled fields written in braces, with no keyword and no declaration site:

fun main() {
    let ev := { user = "7", kind = "click", arg = "/home", line = 1 };
    println("line ${ev.line}: user ${ev.user} did ${ev.kind}");
}

line 1: user 7 did click

{ user: String, kind: String, arg: String, line: i64 } is the type; { user = "7", … } is a value of it. There is no struct Event anywhere — the shape is the type. Two pieces of code that both write { user: String, line: i64 } are talking about the same type without agreeing on it in advance.

Records are:

• structural — { x: i64, y: i64 } and { y: i64, x: i64 } are the same type;
• exact — a { user, kind, arg, line } value is not a { user, kind } value. Records are never silently widened or narrowed (a struct's fields can be, later);
• order-free — { user = "7", line = 1 } and { line = 1, user = "7" } are the same value.

Field access is .name. When a local already has the field's name you can drop the = value: { user, kind } is shorthand for { user = user, kind = kind }.

Wire it into the loop

fun main() {
    let raw := ["7 click /home", "7 buy 42", "3 click /docs"];

    var line := 0;
    for (text in raw) {
        line := line + 1;
        let ev := { text = text, line = line };
        println("line ${ev.line}: ${ev.text}");
    }
}

Each event now carries a line number for error messages — and you didn't declare a type to get it.

By the end you'll have

Each raw line paired with structured context in an anonymous record.

Next: Parse into fields, fail loudly — split the text, and return a Result when a line is malformed.