conditionally-branching-workflows

description: Use predicate-based operators like Effect.filter and Effect.if to declaratively control workflow branching. globs: "**/*.ts" alwaysApply: true

Conditionally Branching Workflows

Rule: Use predicate-based operators like Effect.filter and Effect.if to declaratively control workflow branching.

Example

Here, we use Effect.filterOrFail with named predicates to validate a user before proceeding. The intent is crystal clear, and the business rules (isActive, isAdmin) are reusable.

import { Effect } from "effect";

interface User {
  id: number;
  status: "active" | "inactive";
  roles: string[];
}

type UserError = "DbError" | "UserIsInactive" | "UserIsNotAdmin";

const findUser = (id: number): Effect.Effect<User, "DbError"> =>
  Effect.succeed({ id, status: "active", roles: ["admin"] });

// Reusable, testable predicates that document business rules.
const isActive = (user: User): boolean => user.status === "active";

const isAdmin = (user: User): boolean => user.roles.includes("admin");

const program = (id: number): Effect.Effect<string, UserError> =>
  findUser(id).pipe(
    // Validate user is active using Effect.filterOrFail
    Effect.filterOrFail(isActive, () => "UserIsInactive" as const),
    // Validate user is admin using Effect.filterOrFail
    Effect.filterOrFail(isAdmin, () => "UserIsNotAdmin" as const),
    // Success case
    Effect.map((user) => `Welcome, admin user #${user.id}!`)
  );

// We can then handle the specific failures in a type-safe way.
const handled = program(123).pipe(
  Effect.match({
    onFailure: (error) => {
      switch (error) {
        case "UserIsNotAdmin":
          return "Access denied: requires admin role.";
        case "UserIsInactive":
          return "Access denied: user is not active.";
        case "DbError":
          return "Error: could not find user.";
        default:
          return `Unknown error: ${error}`;
      }
    },
    onSuccess: (result) => result,
  })
);

// Run the program
const programWithLogging = Effect.gen(function* () {
  const result = yield* handled;
  yield* Effect.log(result);
  return result;
});

Effect.runPromise(programWithLogging);