Discriminated union type

[ianduvall]

Is it possible to set up a discriminated union when defining a schema and doing polymorphic queries?

Example:

// .esdl

scalar type ShapeKind extending enum<Circle, Square>;

abstract type Shape {
  require property kind => ShapeKind
}

type Circle extending Shape {
  required property kind -> ShapeKind.Circle
  required property radius -> decimal
}

type Square extending Shape {
  required property kind -> ShapeKind.Circle
  required property sideLength -> decimal
}

// query

e.select(e.Shape, shape => ({
  ...e.is(e.Circle, { ...e.Circle['*']}),
  ...e.is(e.Square, { ...e.Square['*']}),
}));

const result = await query.run(client);
/* (Circle | Square)[] */

[colinhacks]

It's not possible currently, and it's tricky to implement. It's hard to know what the discriminator key should be. I tried to make this work originally, but there are lots of edge cases, e.g. if the type passed into e.is is itself generic:

e.select(e.Shape, shape => ({
  ...e.is(e.Quadrilateral, { ...e.Quadrilateral['*']}),
  ...e.is(e.Square, { ...e.Square['*']}),
}));

You can try using __type__ to differentiate the options at runtime but it won't be reflected in the type.

e.select(e.Shape, shape => ({
  __type__: { name: true },
  ...e.is(e.Quadrilateral, { ...e.Quadrilateral['*']}),
  ...e.is(e.Square, { ...e.Square['*']}),
}));

But the value of __type__.name will never be Quadrilateral, since that's an abstract type, so it wouldn't make sense for this to resolve as a discriminated union. It's a hard problem.