modeling-tagged-unions-with-datacase

description: Use Data.case to define tagged unions (ADTs) for modeling domain-specific states and enabling exhaustive pattern matching. globs: "**/*.ts" alwaysApply: true

Modeling Tagged Unions with Data.case

Rule: Use Data.case to define tagged unions (ADTs) for modeling domain-specific states and enabling exhaustive pattern matching.

Example

import { Data } from "effect";

// Define a tagged union for a simple state machine
type State = Data.TaggedEnum<{
  Loading: {};
  Success: { data: string };
  Failure: { error: string };
}>;
const { Loading, Success, Failure } = Data.taggedEnum<State>();

// Create instances
const state1: State = Loading();
const state2: State = Success({ data: "Hello" });
const state3: State = Failure({ error: "Oops" });

// Pattern match on the state
function handleState(state: State): string {
  switch (state._tag) {
    case "Loading":
      return "Loading...";
    case "Success":
      return `Data: ${state.data}`;
    case "Failure":
      return `Error: ${state.error}`;
  }
}

Explanation:

• Data.case creates tagged constructors for each state.
• The _tag property enables exhaustive pattern matching.
• Use for domain modeling, state machines, and error types.

Explanation:
Modeling domain logic with tagged unions ensures that all cases are handled, prevents illegal states, and enables safe, exhaustive pattern matching.
Data.case provides a concise, type-safe way to define and use ADTs in your application.