effect-io-ai

Package: effect
Module: Effect

Effect.when

Conditionally executes an effect based on a boolean condition.

Details

This function allows you to run an effect only if a given condition evaluates to true. If the condition is true, the effect is executed, and its result is wrapped in an Option.some. If the condition is false, the effect is skipped, and the result is Option.none.

When to Use

This function is useful for scenarios where you need to dynamically decide whether to execute an effect based on runtime logic, while also representing the skipped case explicitly.

Example (Conditional Effect Execution)

import { Effect, Option } from "effect"

const validateWeightOption = (
  weight: number
): Effect.Effect<Option.Option<number>> =>
  // Conditionally execute the effect if the weight is non-negative
  Effect.succeed(weight).pipe(Effect.when(() => weight >= 0))

// Run with a valid weight
Effect.runPromise(validateWeightOption(100)).then(console.log)
// Output:
// {
//   _id: "Option",
//   _tag: "Some",
//   value: 100
// }

// Run with an invalid weight
Effect.runPromise(validateWeightOption(-5)).then(console.log)
// Output:
// {
//   _id: "Option",
//   _tag: "None"
// }

See

• whenEffect for a version that allows the condition to be an effect.
• unless for a version that executes the effect when the condition is false.

Signature

declare const when: { (condition: LazyArg<boolean>): <A, E, R>(self: Effect<A, E, R>) => Effect<Option.Option<A>, E, R>; <A, E, R>(self: Effect<A, E, R>, condition: LazyArg<boolean>): Effect<Option.Option<A>, E, R>; }

Source

Since v2.0.0