Module union

Union shape types and operations.

Defines the shape describing a value drawn from one of several alternatives and its branch types, and provides the union factory stating them and the accessors reading the branches, so that a member whose vocabulary ranges over unrelated types is described by a single shape. Alternatives are stated positionally, and a value is stored as it stands under the one branch it belongs to.

Defining polymorphic members

State the alternatives positionally; cardinality belongs to the enclosing member rather than to a branch:

import { optional, resource } from '@metreeca/blue/resource';
import { reference } from '@metreeca/blue/reference';
import { string } from '@metreeca/blue/string';
import { union } from '@metreeca/blue/union';

const Contact = resource({
  address: optional(union(
    string(),
    reference(PostalAddress)
  ))
});
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Either alternative is accepted at the same position, stored as it stands with no wrapping:

{ "address": "123 Main St" }

{ "address": "https://data.example.com/addresses/456" }
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Reading the alternatives off a shape

The accessors list the alternatives a shape admits values from, treating a shape that is not a union as a single branch, so a caller handles polymorphic and plain shapes alike. They also pick the branches an input is matched against: the one branch a stored value, a relational bound or a set-matching option singles out, and every branch a retrieval placeholder may draw from.