Default Services

Effect comes equipped with five pre-built services:

type DefaultServices = Clock | ConfigProvider | Console | Random | Tracer

When we employ these services, there’s no need to explicitly provide their implementations. Effect automatically supplies live versions of these services to our effects, sparing us from manual setup.

Example (Using Clock and Console)

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import { Effect, Clock, Console } from "effect"
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//      ┌─── Effect<void, never, never>
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//      ▼
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const program = Effect.gen(function* () {
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  const now = yield* Clock.currentTimeMillis
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  yield* Console.log(`Application started at ${new Date(now)}`)
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})
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Effect.runFork(program)
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// Output: Application started at <current time>

As you can observe, even if our program utilizes both Clock and Console, the Requirements parameter, representing the services required for the effect to execute, remains set to never. Effect takes care of handling these services seamlessly for us.

Overriding Default Services

Sometimes, you might need to replace the default services with custom implementations. Effect provides built-in utilities to override these services using Effect.with<service> and Effect.with<service>Scoped.

Effect.with<service>: Overrides a service for the duration of the effect.
Effect.with<service>Scoped: Overrides a service within a scope and restores the original service afterward.

Function	Description
`Effect.withClock`	Executes an effect using a specific `Clock` service.
`Effect.withClockScoped`	Temporarily overrides the `Clock` service and restores it when the scope ends.
`Effect.withConfigProvider`	Executes an effect using a specific `ConfigProvider` service.
`Effect.withConfigProviderScoped`	Temporarily overrides the `ConfigProvider` service within a scope.
`Effect.withConsole`	Executes an effect using a specific `Console` service.
`Effect.withConsoleScoped`	Temporarily overrides the `Console` service within a scope.
`Effect.withRandom`	Executes an effect using a specific `Random` service.
`Effect.withRandomScoped`	Temporarily overrides the `Random` service within a scope.
`Effect.withTracer`	Executes an effect using a specific `Tracer` service.
`Effect.withTracerScoped`	Temporarily overrides the `Tracer` service within a scope.

Example (Overriding Random Service)

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import { Effect, Random } from "effect"
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// A program that logs a random number
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const program = Effect.gen(function* () {
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  console.log(yield* Random.next)
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})
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Effect.runSync(program)
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// Example Output: 0.23208633934454326 (varies each run)
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// Override the Random service with a seeded generator
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const override = program.pipe(Effect.withRandom(Random.make("myseed")))
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Effect.runSync(override)
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// Output: 0.6862142528438508 (consistent output with the seed)