Multi-env App

Read RANK_ENV from the environment and produce an environment-specific configuration by patching a shared base config.

Source

examples/multi-env-app/base.rank

/// Shared base configuration for all environments.

pub AppBase = Object {
  name: string,
  port: number,
  replicas: number,
  logLevel: string,
  db: Object {
    host: string,
    port: number,
    name: string,
    poolSize: number,
  },
}

pub base: AppBase = {
  name: `my-app`,
  port: 8080,
  replicas: 1,
  logLevel: `info`,
  db: {
    host: `localhost`,
    port: 5432,
    name: `app`,
    poolSize: 5,
  },
}

examples/multi-env-app/main.rank

/// Reads RANK_ENV from the environment and produces an environment-specific
/// config by patching the shared base with per-environment overrides.
/// Shows Env inputs, match on a union type, and the `with` override pattern.

use std::Env
use root::base::{ base, AppBase }

SysEnv = Object {
  RANK_ENV?: `dev` | `staging` | `prod` | null,
  ...: string,
}

{ RANK_ENV } = Env<SysEnv> {}

config: AppBase = match RANK_ENV ?? `dev` {
  `dev` => base with {
    logLevel: `debug`,
    db: base.db with { poolSize: 2 },
  },
  `staging` => base with {
    replicas: 2,
    db: base.db with {
      host: `db.staging.internal`,
      poolSize: 10,
    },
  },
  `prod` => base with {
    replicas: 5,
    logLevel: `warn`,
    db: base.db with {
      host: `db.prod.internal`,
      poolSize: 25,
    },
  },
}

pub main = || config

Output

// RANK_ENV=prod
{
  "name": "my-app",
  "port": 8080,
  "replicas": 5,
  "logLevel": "warn",
  "db": {
    "host": "db.prod.internal",
    "port": 5432,
    "name": "app",
    "poolSize": 25
  }
}

Key concepts

• std::Env — reads environment variables into a typed schema. RANK_ENV is constrained at the boundary to `dev` | `staging` | `prod` | null instead of being normalized later from an arbitrary string.
• Destructuring env inputs — { RANK_ENV } = Env<SysEnv> {} pulls the field you care about into a local binding while keeping the schema on the Env call.
• match on a literal union — match RANK_ENV ?? dev`` branches over the closed set of allowed environments with an explicit default when the variable is unset.
• Nested with — base with { db: base.db with { poolSize: 25 } } patches deeply without touching unrelated fields.
• Multi-file modules — base.rank exports the shared type and value; this example imports them via the canonical absolute form root::base. In a normal project source tree, a nearby sibling import can also use super::base when that reads better.

Run it

RANK_ENV=prod rank examples/multi-env-app
RANK_ENV=staging rank examples/multi-env-app
rank examples/multi-env-app   # defaults to dev