control-flow-with-combinators.mdx

title	Control Flow with Conditional Combinators
id	control-flow-with-combinators
skillLevel	intermediate
applicationPatternId	core-concepts
summary	Use combinators like Effect.if, Effect.when, and Effect.cond to handle conditional logic in a declarative, composable way.
tags	control-flow conditional if when cond declarative
rule	description Use conditional combinators for control flow.
related	use-gen-for-business-logic
author	Sandro Maglione
lessonOrder	3

Control Flow with Conditional Combinators

Guideline

Use declarative combinators like Effect.if, Effect.when, and Effect.unless to execute effects based on runtime conditions.

Rationale

These combinators allow you to embed conditional logic directly into your .pipe() compositions, maintaining a declarative style for simple branching.

Good Example

import { Effect } from "effect";

const attemptAdminAction = (user: { isAdmin: boolean }) =>
  Effect.if(user.isAdmin, {
    onTrue: () => Effect.succeed("Admin action completed."),
    onFalse: () => Effect.fail("Permission denied."),
  });

const program = Effect.gen(function* () {
  // Try with admin user
  yield* Effect.logInfo("\nTrying with admin user...");
  const adminResult = yield* Effect.either(
    attemptAdminAction({ isAdmin: true })
  );
  yield* Effect.logInfo(
    `Admin result: ${adminResult._tag === "Right" ? adminResult.right : adminResult.left}`
  );

  // Try with non-admin user
  yield* Effect.logInfo("\nTrying with non-admin user...");
  const userResult = yield* Effect.either(
    attemptAdminAction({ isAdmin: false })
  );
  yield* Effect.logInfo(
    `User result: ${userResult._tag === "Right" ? userResult.right : userResult.left}`
  );
});

Effect.runPromise(program);

Explanation:
Effect.if and related combinators allow you to branch logic without leaving the Effect world or breaking the flow of composition.

Anti-Pattern

Using Effect.gen for a single, simple conditional check can be more verbose than necessary. For simple branching, Effect.if is often more concise.