Type relations

Type hierarchy

Model with properties

When determining if type S can be assigned to type T, if T is a model with properties, it checks whether all those properties are present in S and if their types can be assigned to the type of the corresponding property in T.

For instance,

model T {
  foo: string;
  bar: int32;
}

// Valid

model S { // When properties types are the exact same
  foo: string;
  bar: int32;
}
model S { // When the properties types are literal assignable to the target type
  foo: "abc";
  bar: 123;
}
model S {
  foo: string;
  bar: int8; // int8 is assignable to int16
}
model S {
  foo: string;
  bar: int32;
  otherProp: boolean; // Additional properties are valid.
}

// Invalid
model S { // Missing property bar
  foo: string;
}
model S {
  foo: string;
  bar: int64; // int64 is NOT assignable to int32
}

Record<T>

A record is a model indexed with a string with a value of T. It represents a model where all properties (string keys) are assignable to the type T. You can assign a model expression where all the properties are of type T or another model that is also a Record<T>.

// Represent an object where all the values are int32.
alias T = Record<int32>;

// Valid
alias S = {
  foo: 123;
  bar: 345;
};
alias S = {
  foo: int8;
  bar: int32;
};
model S is Record<int32>;
model S is Record<int32> {
  foo: 123;
}

// Invalid
alias S = {
  foo: "abc";
  bar: 456;
};
alias S = {
  foo: int64;
  bar: int32;
};
model S {
  foo: 123;
  bar: 456;
}

Why isn't the last case assignable to Record<int32>?

In this scenario,

alias T = Record<int32>;
model S {
  foo: 123;
  bar: 456;
}

The reason why model S is not assignable, but the model expression { foo: 123; bar: 456; } is, is because model S could be extended with additional properties that might not be compatible.

For instance, if you add a new model,

model Foo is S {
  otherProp: string;
}

Here, Foo is assignable to S following the model with property logic, and if S was assignable to Record<int32>, Foo would also be passable. However, this is now invalid as otherProp is not an int32 property.