Configuration in Effect

Overview

Effect provides type-safe configuration loading with:

• Automatic environment variable reading
• Validation and type conversion
• Default values and composition
• Sensitive value handling
• Multiple config sources (env, JSON, custom)

Basic Configuration Types

import { Config, Effect } from "effect"

const host = Config.string("HOST")

const port = Config.number("PORT")

const debug = Config.boolean("DEBUG")

const maxConnections = Config.integer("MAX_CONNECTIONS")

Using Config in Effects

const program = Effect.gen(function* () {
  const host = yield* Config.string("DATABASE_HOST")
  const port = yield* Config.number("DATABASE_PORT")

  return { host, port }
})

// Runs and reads from environment
await Effect.runPromise(program)

Default Values

const port = Config.number("PORT").pipe(
  Config.withDefault(3000)
)

const debug = Config.boolean("DEBUG").pipe(
  Config.withDefault(false)
)

Optional Configuration

const apiKey = Config.string("API_KEY").pipe(
  Config.option
)
// Type: Effect<Option<string>>

Combining Configurations

Using Config.all

const dbConfig = Config.all({
  host: Config.string("DB_HOST"),
  port: Config.number("DB_PORT"),
  database: Config.string("DB_NAME"),
  maxConnections: Config.number("DB_MAX_CONN").pipe(
    Config.withDefault(10)
  )
})

const program = Effect.gen(function* () {
  const config = yield* dbConfig
  // config: { host: string, port: number, database: string, maxConnections: number }
})

Nested Configurations

const dbConfig = Config.nested("DB")(
  Config.all({
    host: Config.string("HOST"),      // Reads DB_HOST
    port: Config.number("PORT"),      // Reads DB_PORT
    name: Config.string("NAME")       // Reads DB_NAME
  })
)

Config with Schema Validation

Use Effect Schema for complex validation:

import { Config, Schema } from "effect"

const PortSchema = Schema.Number.pipe(
  Schema.int(),
  Schema.between(1, 65535)
)

const port = Config.number("PORT").pipe(
  Config.mapOrFail((n) =>
    Schema.decodeUnknownEither(PortSchema)(n).pipe(
      Either.mapLeft((e) => ConfigError.InvalidData([], `Invalid port: ${n}`))
    )
  )
)

Handling Sensitive Values

Config.redacted

Prevents accidental logging of sensitive values:

const apiKey = Config.redacted("API_KEY")
// Type: Effect<Redacted<string>>

const program = Effect.gen(function* () {
  const key = yield* apiKey

  // Safe to log - shows "<redacted>"
  yield* Effect.log(`Key: ${key}`)

  // Get actual value when needed
  const actual = Redacted.value(key)
})

Secret Type

const dbPassword = Config.secret("DB_PASSWORD")
// Type: Effect<Secret.Secret>

const program = Effect.gen(function* () {
  const password = yield* dbPassword
  const value = Secret.value(password) // Get actual string
})

Config Operators

Transforming Values

const upperHost = Config.string("HOST").pipe(
  Config.map((s) => s.toUpperCase())
)

const port = Config.string("PORT").pipe(
  Config.mapOrFail((s) => {
    const n = parseInt(s)
    return isNaN(n)
      ? Either.left(ConfigError.InvalidData([], "Not a number"))
      : Either.right(n)
  })
)

Fallback Values

const host = Config.string("PRIMARY_HOST").pipe(
  Config.orElse(() => Config.string("SECONDARY_HOST")),
  Config.orElse(() => Config.succeed("localhost"))
)

Custom Config Providers

From Environment (Default)

const program = Effect.gen(function* () {
  const host = yield* Config.string("HOST")
})

From JSON/Object

import { ConfigProvider, Layer } from "effect"

const config = {
  host: "localhost",
  port: "3000",
  database: {
    host: "db.example.com",
    port: "5432"
  }
}

const JsonConfigProvider = ConfigProvider.fromJson(config)

const program = Effect.gen(function* () {
  const host = yield* Config.string("host")
  const dbHost = yield* Config.nested("database")(Config.string("host"))
})

const runnable = program.pipe(
  Effect.provide(Layer.setConfigProvider(JsonConfigProvider))
)

From Map

const MapProvider = ConfigProvider.fromMap(
  new Map([
    ["HOST", "localhost"],
    ["PORT", "3000"]
  ])
)

Combining Providers

const CombinedProvider = ConfigProvider.orElse(
  ConfigProvider.fromEnv(),
  () => ConfigProvider.fromJson(defaultConfig)
)

Config in Layers

const AppConfigLive = Layer.effect(
  AppConfig,
  Effect.gen(function* () {
    const host = yield* Config.string("HOST")
    const port = yield* Config.number("PORT")
    const debug = yield* Config.boolean("DEBUG").pipe(Config.withDefault(false))

    return { host, port, debug }
  })
)

Testing Configuration

Mock Config Provider

const TestConfigProvider = ConfigProvider.fromMap(
  new Map([
    ["HOST", "test-host"],
    ["PORT", "9999"]
  ])
)

const testProgram = program.pipe(
  Effect.provide(Layer.setConfigProvider(TestConfigProvider))
)

Config.succeed for Hardcoded

const testConfig = Config.succeed({
  host: "localhost",
  port: 3000
})

Error Handling

Config failures produce ConfigError:

const program = Effect.gen(function* () {
  const host = yield* Config.string("REQUIRED_HOST")
}).pipe(
  Effect.catchTag("ConfigError", (error) =>
    Effect.fail(new StartupError({ cause: error }))
  )
)

Complete Example

import { Config, Effect, Layer, Schema } from "effect"

// Define config shape
const AppConfig = Config.all({
  server: Config.nested("SERVER")(
    Config.all({
      host: Config.string("HOST").pipe(Config.withDefault("0.0.0.0")),
      port: Config.number("PORT").pipe(Config.withDefault(3000))
    })
  ),
  database: Config.nested("DATABASE")(
    Config.all({
      url: Config.redacted("URL"),
      maxConnections: Config.number("MAX_CONN").pipe(Config.withDefault(10))
    })
  ),
  features: Config.all({
    debug: Config.boolean("DEBUG").pipe(Config.withDefault(false)),
    metrics: Config.boolean("METRICS_ENABLED").pipe(Config.withDefault(true))
  })
})

// Use in application
const program = Effect.gen(function* () {
  const config = yield* AppConfig
  yield* Effect.log(`Starting server on ${config.server.host}:${config.server.port}`)
})

Best Practices

1. Use Config.withDefault for optional values – Avoid runtime errors
2. Use Config.redacted for secrets – Prevents accidental logging
3. Use Config.nested for structure – Organizes related config
4. Validate with Schema – Catch invalid config early
5. Test with mock providers – Deterministic tests

Additional Resources

For comprehensive configuration documentation, consult ${CLAUDE_PLUGIN_ROOT}/references/llms-full.txt.

Search for these sections:

• “Configuration” for full API reference
• “ConfigProvider” for custom providers
• “Handling Sensitive Values” for security