Traits

Traits are typed annotations you attach above a shape or member with @name(...). They add metadata and constraints without changing the structural type. You’ve already met one: @required.

Traits stack — one per line:

@documentation("A two-to-fifty character city name.")
@length(min: 2, max: 50)
@pattern("^[A-Za-z ]+$")
string CityName

Constraint traits: target matters

The two most common constraints apply to different kinds of shapes:

• @length(min, max) — the size of a string, blob, list, or map.
• @range(min, max) — the numeric value of a number shape.

@length(min: 1, max: 100)
string Title

@range(min: 0, max: 130)
integer Age

Applying @range to a string, or @length to an integer, is a target mismatch the compiler rejects.

@pattern matches substrings

@pattern takes an ECMA-262 regex — but it matches anywhere in the value unless you anchor it. Use ^...$ when you mean the whole string:

@pattern("^[A-Za-z ]+$")
string CityName

Traits on members, too

structure Person {
    @required
    @documentation("Display name.")
    name: String
}

Your turn