Shapes

A shape describes the keys of a record, in order. It is the record’s type. All records with the same keys in the same order share exactly one shape value.

a = { name: 'Joe' }
b = { name: 'Rich' }

a?          # => <gab\shape name:>
b?          # => <gab\shape name:>
(a?) == (b?)  # => true:

A space is required between an identifier and ?. Names in Gab may end with ? or !, so a? would be parsed as a single identifier, not a message send.

Obtaining a Shape

There are two ways to get a shape value.

From a record, using the ? message:

Person = { name: '', age: 0 }?
# => <gab\shape name: age:>

Directly, using Shapes.make: with a list of keys:

Person = Shapes.make(name:, age:)
# => <gab\shape name: age:>

Both produce the same shape value. The ? approach is convenient when you already have an example record; Shapes.make is useful when you want to define the shape without constructing a record first.

Shapes as Specialization Targets

Shapes are most useful as receiver types in def:, defcase:, and defmodule:. All records with a matching shape will respond to the defined message:

Person = Shapes.make(name:, age:)

birthday: .def (Person, () => do
  'Happy Birthday, $!'.sprintf(self.name).println
  self.put(age: self.age + 1)
end)

bob = { name: 'bob', age: 44 }
bob = bob.birthday
# => Happy Birthday, bob!

bob.age   # => 45

Shapes in Dispatch

When Gab resolves a message send, the shape is checked as the super type before the record’s base type (gab\record). This means shape-specialised messages take precedence over general record behaviour:

z: .def (Shapes.make(x:), 'shape case')
z: .def 'general case'

{ x: 1 }.z   # => 'shape case'   (shape wins over general)
{ y: 1 }.z   # => 'general case'

See Messages — Dispatch Resolution Order for the full sequence.