Default Services

Effect comes equipped with live implementations of five services: Clock, ConfigProvider, Console, Random, and 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>
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await Effect.runPromise(program) // => undefined

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

Each default service is exposed as a Context.Reference: Clock.Clock, ConfigProvider.ConfigProvider, Console.Console, Random.Random, and Tracer.Tracer. Use Effect.provideService to run an effect with a different implementation. The override applies only to the effect being provided.

Some modules also provide higher-level helpers for common overrides. For example, Random.withSeed installs a deterministic random-number generator for an effect.

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(Random.withSeed("myseed"))
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Effect.runSync(override)
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// Output: 0.10428056576185751 (consistent output with the seed)
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// The seed makes the generated value fully deterministic
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Effect.runSync(Random.next.pipe(Random.withSeed("myseed"))) // => 0.10428056576185751