Runtime without upfront layer

[lucas-barake]

I'd like to define atoms with service dependencies in a shared package and provide different layer implementations per platform (e.g. web vs native).

Currently Atom.runtime(layer) requires the layer at definition time, which couples the atom definitions to a specific implementation.

Would something like this make sense?

// core/runtime.ts - create runtime without layer
export const runtime = Atom.lazyRuntime<Api | Storage>()

// core/atoms.ts - real atoms with real dependencies, defined once
export const users = runtime.atom(Effect.flatMap(Api, a => a.getUsers()))
export const posts = runtime.atom(Effect.flatMap(Api, a => a.getPosts()))
export const userCount = Atom.make((get) => Result.map(get(users), u => u.length))

// web/main.tsx - provide layer
<RegistryProvider initialValues={[Atom.initialValue(runtime.layer, WebApiLayer)]}>

// native/main.tsx - different layer, same atoms
<RegistryProvider initialValues={[Atom.initialValue(runtime.layer, NativeApiLayer)]}>

[tim-smart]

It won't be type safe as you can't enforce that an initial value is supplied.

Currently, you can override the layer using runtime.layer for tests etc.

https://github.com/Effect-TS/effect-smol/blob/main/packages%2Fatom%2Freact%2Ftest%2Findex.test.tsx#L40

[lucas-barake]

Yeah, makes sense. In that case, what would you recommend in user-land?

I was thinking of something like this:

const lazyRuntime = <R>() => {
  const layerSlot = Atom.make(undefined as Layer.Layer<R> | undefined)
  const rt = Atom.runtime((get) => {
    const layer = get(layerSlot)
    if (layer === undefined) {
      throw new Error("Runtime layer not provided")
    }
    return layer
  })
  return Object.assign(rt, { layerSlot })
}

// shared
const runtime = lazyRuntime<Api | Storage>()
export const users = runtime.atom(Effect.flatMap(Api, (a) => a.getUsers()))
export const posts = runtime.atom(Effect.flatMap(Api, (a) => a.getPosts()))

// web/main.tsx
<RegistryProvider initialValues={[Atom.initialValue(runtime.layerSlot, WebApiLayer)]}>
  <App />
</RegistryProvider>

// native/main.tsx
<RegistryProvider initialValues={[Atom.initialValue(runtime.layerSlot, NativeApiLayer)]}>
  <App />
</RegistryProvider>

[tim-smart]

Potentially you could just do a Layer.effect(tag, Effect.die("unimplemented")) for the Atom.runtime.