Named

Use ft.Named to add names to types for use in error messages. This is especially useful for large unions that can otherwise have very long error messages.

import * as ft from "funtypes";

const UserCodec = ft.Named(
  "User",
  ft.Object({
    type: ft.Literal("USER"),
    id: ft.Number,
    name: ft.String,
  }),
);

const PostCodec = ft.Named(
  "Post",
  ft.Object({
    type: ft.Literal("POST"),
    id: ft.Number,
    title: ft.String,
  }),
);

export const DbObjectCodec = ft.Union(
  UserCodec,
  PostCodec,
);
// => ft.Codec<{ type: "USER"; id: number; name: string } | { type: "POST"; id: number; title: string }>

export type DbObjectType = ft.Static<
  typeof DbObjectCodec
>;
// => { type: "USER"; id: number; name: string } | { type: "POST"; id: number; title: string }

// ✅ Instead of printing out the full type, it
//    just refers to sub-types by name
assert.deepEqual(
  ft.showType(DbObjectCodec),
  "User | Post",
);

Other than improving error messages, ft.Named should never make any difference to the behaviour of your funtypes.