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Lesson 6

Traits

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

Your turn

Now the checks get sharper: a hasTrait check can assert a trait’s exact value. So it’s not enough to put some @length on CityName — it must be min: 2, max: 50, and Age’s @range must be min: 0, max: 130.