Configuration

Reading config from the environment and validating it is the most common fallible construction in a real app — and it's just a fallible Layer.make fed by a schema. demesne adds no config primitive of its own (that would break "does one thing: wiring"). The schema → Result bridge already lives in unthrown's ecosystem; demesne only wires the validated result.

With @unthrown/standard-schema

@unthrown/standard-schema turns any Standard Schema (Zod, Valibot, ArkType, …) into an unthrown Result. Compose it inside Layer.make.

Inject the raw environment as a port rather than reaching for process.env inside the layer — it keeps config testable (a fake env in tests, the real one at the edge) and is the boundary-declared style demesne favours.

import { type Context, Layer, Tag } from "demesne";
import { fromSchema, type SchemaIssues } from "@unthrown/standard-schema";
import { type Result, TaggedError } from "unthrown";
import { z } from "zod"; // any Standard Schema validator

// The raw environment is a provided port.
class Env extends Tag("Env")<Env, Record<string, string | undefined>>() {}

const ConfigSchema = z.object({ dbUrl: z.string().url() });
class AppConfig extends Tag("AppConfig")<AppConfig, z.infer<typeof ConfigSchema>>() {}

// A modeled, discriminated error for the E channel (nicer at the edge than a raw
// issues array). Drop the `mapErr` if `SchemaIssues` is fine for you.
class ConfigError extends TaggedError("ConfigError")<{ issues: SchemaIssues }> {}

const AppConfigLive = Layer.make(AppConfig, (ctx: Context<Env>) =>
  fromSchema(ConfigSchema)(ctx.get(Env)).mapErr((issues) => new ConfigError({ issues })),
);
//    ^? Layer<AppConfig, ConfigError, Env>

// Wire the env at the composition edge.
const result = await Layer.build(Layer.provideTo(AppConfigLive, Layer.value(Env, process.env)));
//    ^? Result<Context<AppConfig>, ConfigError>

Use fromSchemaAsync instead if your schema validates asynchronously — it returns an AsyncResult, which Layer.make accepts unchanged.

Where sugar belongs

If you find yourself repeating this trio, it promotes cleanly into a thin @demesne/standard-schema adapter package (the monorepo is built to grow that way) — but it does not belong in the core.