<+> is an alias to function orElseEither. This is used to represent fall-back logic when we describe config retrievals. Unlike orElse, the the fall-back config parameter can have a different type in orElseEither.

Example:

val config: ConfigDescriptor[Either[Int, String]] = int("MONTH") <+> string("MONTH")

This is a description that represents the following: Try to retrieve the value of a MONTH as an Int, and if there is a format error, try and retrieve it as a String.

Detail:

We know ConfigDescriptor is a program that describes the retrieval of a set of configuration parameters. In the below example, we can either depend on a configuration called password or a token both being of the same type, in this case, a String.

Example:

Given:

final case class BasicAuth(username: String, password: String)
final case class OAuth(clientId: String, secret: String)

val basicAuth: ConfigDescriptor[BasicAuth] =
  (string("USERNAME") zip string("PASSWORD")).to[BasicAuth]

val oAuth: ConfigDescriptor[OAuth] =
  (string("CLIENT_ID") zip string("SECRET")).to[OAuth]

val myConfig: ConfigDescriptor[Either[BasicAuth, OAuth]] =
  basicAuth <+> oAuth

then,

val source = ConfigSource.fromMap(Map("USERNAME" -> "abc", "PASSWORD" -> "cde")

read(myConfig from source)

returns:

Left(BasicAuth("abc", "def")

Similarly,

val source = ConfigSource.fromMap(Map("CLIENT_ID" -> "xyz", "SECRET" -> "afg==")

read(myConfig from source)

returns:

Right(OAuth("xyz", "afg==")

<> is an alias to function orElse. This is used to represent fall-back logic when we describe config retrievals.

Example:

val config: ConfigDescriptor[String] = string("token") <> string("password")

This is a description that represents the following: Try to retrieve the value of a parameter called "token", or else try to retrieve the value of parameter called "password"

We know ConfigDescriptor is a program that describes the retrieval of a set of configuration parameters.

In the below example, we can either depend on a configuration called password or a token both being of the same type, in this case, a String.

Example:

final case class Config(tokenOrPassword: String, port: Int)

object Config {
  val databaseConfig: ConfigDescriptor[Config] =
    (string("token") <> string("password")  zip int("PORT")).to[Config]
}

?? is an alias to describe which allows us to inject additional documentation to the configuration parameters.

Example:

val port = int("PORT") ?? "database port"

A more detailed example:

Here is a program that describes (or a ConfigDescriptor that represents) reading a USERNAME which is a String and PORT which is an Int, and load it to a case class Config

final case class Config(userName: String, port: Int)

object Config {
   val dbConfig: ConfigDescriptor[Config] =
      (string("USERNAME") zip int("PORT")).to[Config]
}

Later on you decided to annotate each one of them with extra documentation, which is later seen in error messages if config retrieval is a failure, and it's also used while documenting your configuration using ConfigDocsModule

val dbConfigWithDoc: ConfigDescriptor[Config] =
   (string("USERNAME") ?? "db username" zip
      int("PORT") ?? "db port"
     ).to[Config]

If you try and read this config from an empty source, it emits an error message with the details you provided.

import zio.config._, ConfigDescriptor._
read(Config.databaseConfig from ConfigSource.fromMap(Map.empty))

returns:

╥
╠══╦══╗
║  ║  ║
║  ║  ╠─MissingValue
║  ║  ║ path: PORT
║  ║  ║ Details: db port, value of type int
║  ║  ▼
║  ║
║  ╠─MissingValue
║  ║ path: USERNAME
║  ║ Details: db username, value of type string
║  ▼
▼

Or, you can also use a common documentation for an entire set of config parameters.

val detailedConfigDescriptor: ConfigDescriptor[Config] =
  configDescriptor ?? "Configuration related to database"

default function allows us to inject default values to existing config

Example:

val port = int("PORT").default(8080)

A more detailed example:

Here is a program that describes (or a ConfigDescriptor that represents) reading a USERNAME which is a String and PORT which is an Int, and load it to a case class Config

final case class Config(userName: String, port: Int)

object Config {
   val dbConfig: ConfigDescriptor[Config] =
      (string("USERNAME") zip int("PORT").default(8080)).to[Config]
}

In the above case, if username is missing, then it prints out an error, however if PORT is missing, it falls back to 8080.

In fact you can give a default to an entire config

For example:

 final case class Config(userName: String, port: Int)

object Config {
   val dbConfig: ConfigDescriptor[Config] =
      (string("USERNAME") zip int("PORT")).to[Config].default(Config("jon", 8080))
}

Sometimes this can be used along with automatic derivation supported through zio-config-magnolia.

import zio.config.magnolia._, zio.config._, ConfigDescriptor._

final case class Config(userName: String, port: Int)

object Config {
  val dbConfig: ConfigDescriptor[Config] =
    descriptor[Config].default(Config("jon", 8080))
}

// This is a typical example where we mix auto derivation with manual definitions.

describe function allows us to inject additional documentation to the configuration parameters.

Example:

val port = int("PORT") ?? "database port"

A more detailed example:

Here is a program that describes (or a ConfigDescriptor that represents) reading a USERNAME which is a String and a PORT which is an Int, and load it to a case class Config

final case class Config(userName: String, port: Int)

object Config {
   val dbConfig: ConfigDescriptor[Config] =
      (string("USERNAME") zip int("PORT")).to[Config]
}

Later on you decided to annotate each one of them with extra documentation, which is later seen in error messages if config retrieval is a failure, and it's also used while documenting your configuration using ConfigDocsModule

val dbConfigWithDoc: ConfigDescriptor[Config] =
   (string("USERNAME") ?? "db username" zip int("PORT") ?? "db port").to[Config]

If you try and read this config from an empty source, it emits an error message with the details you provided.

import zio.config._, ConfigDescriptor._

read(Config.databaseConfig from ConfigSource.fromMap(Map.empty))

returns:

╥
╠══╦══╗
║  ║  ║
║  ║  ╠─MissingValue
║  ║  ║ path: PORT
║  ║  ║ Details: db port, value of type int
║  ║  ▼
║  ║
║  ╠─MissingValue
║  ║ path: USERNAME
║  ║ Details: db username, value of type string
║  ▼
▼

Or, you can also use a common documentation for an entire set of config parameters.

val detailedConfigDescriptor: ConfigDescriptor[Config] =
  configDescriptor ?? "Configuration related to database"

Attach a source to the ConfigDescriptor.

Example:

val config = string("PORT") from ConfigSource.fromMap(Map.empty)

config is a description that says there is a key called PORT in constant map source. You can use the description to read the config

val either: ZIO[Any, ReadError[String], String] = read(config)

You can also tag a source per config field, or one global source to an entire config.

final case class Config(userName: String, port: Int)
object Config {
   val dbConfig: ConfigDescriptor[Config] =
      (string("USERNAME") zip int("PORT")).to[Config]
}

In the above example, dbConfig is not associated with any source. By default the source will be empty.

To attach a config (especially during a read operation) is as easy as:

read(dbConfig from ConfigSource.fromMap(Map("USERNAME" -> "afs", "PORT" -> "8080"))
// Right(Config("afs", 8080))

Obviously, source can be attached independently.

val configSource1: ConfigSource = ???
val configSource2: ConfigSource = ???

val dbConfig =
  (string("USERNAME") from configSource1 zip int("PORT")).to[Config] from configSource2

In the above case read(dbConfig) implies, zio-config tries to fetch USERNAME from configSource1, and if it fails (i.e, missing value) it goes and try with the global config which is configSource2. PORT will be fetched from configSource2.

You can also try various sources for each field.

val configSource1: ConfigSource = ??? // Example: ConfigSource.fromMap(...)
val configSource2: ConfigSource = ??? // Example: ConfigSource.fromTypesafeConfig(...)

val dbConfig =
  (string("USERNAME") from configSource1.orElse(configSource2) zip
    int("PORT") from configSource2.orElse(configSource1)).to[Config] from configSource2

Transform A to B, however usage of map disallows the usage of write back functionality.

Use map if we really don't care writing back the config to the sources in future

Transform A to B that can fail, however usage of mapEither disallows the usage of write back functionality.

Use mapEither if we really don't care writing back the config to the sources in future

mapKey allows user to convert the keys in a ConfigDescriptor.

Example:

Consider you have a config that looks like this

 case class Config(url: String, port: Int)

 object Config {

    val config = (string("dbUrl") zip int("dbPort")).to[Config]
 }

val source = Map(
   "DB_URL" -> "abc.com",
   "DB_PORT" -> "9090"
)

read(Config.config from ConfigSource.fromMap(source))
// will fail since the source doesn't have the keys dbUrl and dbPort, but it has only DB_URL and DB_PORT

The above config retrieval fails since the keys dbUrl and dbPOrt exist, but it has only DB_URL and DB_PORT. In this situation, instead of rewriting the config we can do

import zio.config._, ConfigDescriptor._

read(Config.config.mapKey(key => toSnakeCase(key).toUpperCase) from ConfigSource.fromMap(source))
// Right(Config("abc.com", 9090))

optional function allows us to tag a configuration parameter as optional. It implies, even if it's missing configuration will be a success.

Example:

val port: ConfigDescriptor[Option[Int]] = int("PORT").optional

A more detailed example:

Here is a program that describes (or a ConfigDescriptor that represents) reading a USERNAME which is a String and PORT which is an Int, and load it to a case class Config

final case class Config(userName: String, port: Option[Int])

object Config {
   val dbConfig: ConfigDescriptor[Config] =
      (string("USERNAME") zip int("PORT").optional).to[Config]
}

The fact that it is an optional in error messages if config retrieval is a failure, and it's also used while documenting your configuration using ConfigDocsModule

val dbConfigWithDoc: ConfigDescriptor[Config] =
   (string("USERNAME") ?? "db username" zip int("PORT") ?? "db port").to[Config]

import zio.config._, ConfigDescriptor._

val source = ConfigSource.fromMap(Map("USERNAME" -> "af"))

read(Config.databaseConfig from source)

returns:

Config("af", None)

Similarly,

val source = ConfigSource.fromMap(Map("USERNAME" -> "af", "PORT" -> "8888"))

read(Config.databaseConfig from source)

returns:

Config("af", Some(8888))

However, if you have given PORT, but it's not an integer, then it fails giving you the error details.

Within the error message, it will also specify the fact that the parameter is an optional parameter, giving you an indication that you can either fix the parameter, or you can completely skip this parameter.

Example:

import zio.config._, ConfigDescriptor._

val source = ConfigSource.fromMap(Map("USERNAME" -> "af", "PORT" -> "abc"))

read(Config.databaseConfig from source)

returns:

╥
╠══╗
║  ║
║  ╠─FormatError
║  ║ cause: Provided value is abc, expecting the type int
║  ║ path: PORT
║  ▼
▼

Another interesting behaviour, but we often forget about optional parameters is when there is a presence of a part of the set of the config parameters representing a product, where the product itself is optional.

Example:

final case class DbConfig(port: Int, host: String)

object DbConfig {
  val dbConfig: ConfigDescriptor[Option[DbConfig]] =
    (int("PORT") zip string("HOST")).to[DbConfig].optional
}

In this case if "PORT" is present in the source, but "HOST" is absent, then config retrieval will be a failure and not None. Similarly, if "HOST" is present but "PORT" is absent, the config retrieval will be a failure and not None.

If both of the parameters are absent in the source, then the config retrieval will be a success and the output will be None. If both of them is present, then output will be Some(DbConfig(..))

orElse is used to represent fall-back logic when we describe config retrievals.

Example:

val config: ConfigDescriptor[String] = string("token") <> string("password")

This is a description that represents the following: Try to retrieve the value of a parameter called "token", or else try to retrieve the value of parameter called "password"

We know ConfigDescriptor is a program that describes the retrieval of a set of configuration parameters. In the below example, we can either depend on a configuration called password or a token both being of the same type, in this case, a String.

Example:

final case class Config(tokenOrPassword: String, port: Int)

object Config {
  val databaseConfig: ConfigDescriptor[Config] =
    (string("token") <> string("password")  zip int("PORT")).to[Config]
}

Note: orElse is different from orElseEither.

While orElse fall back to parameter which is of the same type of the original config parameter, orElseEither can fall back to a different type giving us Either[A, B].

orElse will be useful in retrieving configuration that are represented as coproducted (sealed trait). However, it may become fairly verbose, such that usage zio-config-magnolia to derive the config automatically, will become a reasonable alternative.

orElseEither is used to represent fall-back logic when we describe config retrievals. Unlike orElse, the fall-back config parameter can have a different type in orElseEither.

Example:

val config: ConfigDescriptor[Either[Int, String]] = int("MONTH") <+> string("MONTH")

This is a description that represents the following: Try to retrieve the value of a MONTH as an Int, and if there is a format error, try and retrieve it as a String.

Detail:

We know ConfigDescriptor is a program that describes the retrieval of a set of configuration parameters. In the below example, we can either depend on a configuration called password or a token both being of the same type, in this case, a String.

Example:

Given:

final case class BasicAuth(username: String, password: String)
final case class OAuth(clientId: String, secret: String)

val basicAuth: ConfigDescriptor[BasicAuth] =
  (string("USERNAME") zip string("PASSWORD")).to[BasicAuth]

val oAuth: ConfigDescriptor[OAuth] =
  (string("CLIENT_ID") zip string("SECRET")).to[OAuth]

val myConfig: ConfigDescriptor[Either[BasicAuth, OAuth]] =
  basicAuth <+> oAuth

then,

val source = ConfigSource.fromMap(Map("USERNAME" -> "abc", "PASSWORD" -> "cde")

read(myConfig from source)

returns:

Left(BasicAuth("abc", "def")

Similarly,

val source = ConfigSource.fromMap(Map("CLIENT_ID" -> "xyz", "SECRET" -> "afg==")

read(myConfig from source)

returns:

Right(OAuth("xyz", "afg==")

Convert a ConfigDescriptor[A] to a config descriptor of a case class

This works when A is a single value and B is a single parameter case class with the same type of parameter, or if A is an tuple and B is a case class with matching number of parameters and the same types.

See the following example of reading a USERNAME which is a String and PORT which is an Int, and load it to a case class Config:

final case class Config(userName: String, port: Int)

object Config {
   val dbConfig: ConfigDescriptor[Config] = (string("USERNAME") zip int("PORT")).to[Config]
}

Given A and B, f: A => B, and g: B => A, then transform allows us to transform a ConfigDescriptor[A] to ConfigDescriptor[B].

Example : transform is useful especially when you define newtypes.

final case class Port(port: Int) extends AnyVal

val config: ConfigDescriptor[Port] =
  int("PORT").transform[Port](Port.apply, _.int)

While to: A => B (in this case, Int => Port) is used to read to a Port case class, from: B => A (which is, Port => Int) is used when we want to write Port directly to a source representation.

Example:

import zio.config.typesafe._ // as toJson is available only through zio-config-typesafe module

val writtenBack: Either[String, PropertyTree[String, String]] = write(config, Port(8888))

val jsonRepr: Either[String, String] = writtenBack.map(_.toJson) // { "port" : "8888" }
val mapRepr: Either[String, Map[String, String]] = writtenBack.map(_.flattenString()) // Map("port" -> "8888")

Given A and B, transformOrFail function is used to convert a ConfigDescriptor[A] to ConfigDescriptor[B].

It is important to note that both to and fro is fallible, allowing us to represent almost all possible relationships.

Example:

Let's define a simple ConfigDescriptor, that talks about retrieving a S3Path ( a bucket and prefix in AWS s3). Given you want to retrieve an S3Path from ConfigSource. Given a string, converting it to S3Path can fail, and even converting S3Path to a String can fail as well.

import java.time.DateTimeFormatter
import java.time.LocalDate

final case class S3Path(bucket: String , prefix: String, partition: LocalDate) {
  def convertToString(partitionPattern: String): Either[String, String] =
    Try { DateTimeFormatter.ofPattern(partitionPattern).format(partition) }.toEither
      .map(dateStr => s"${bucket}/${prefix}/${dateStr}").swap.map(_.getMessage).swap
}

object S3Path {
  def fromStr(s3Path: String): Either[String, S3Path] = {
    val splitted = s3Path.split("/").toList

    if (splitted.size > 3)
      Left("Invalid s3 path")
    else
      for {
         bucket <- splitted.headOption.toRight("Empty s3 path")
         prefix <- splitted.lift(1).toRight("Invalid prefix, or empty prefix in s3 path")
         partition <- splitted.lift(2).toRight("Empty partition").flatMap(dateStr => LocalDate.parse(dateStr))
      } yield S3Path(bucket, prefix, partition)
  }
}

val s3PathConfig: ConfigDescriptor[S3Path] =
  string("S3_PATH").transformEither[S3Path](S3Path.fromStr, _.convertToString("yyyy-MM-dd"))

Untag all sources associated with a ConfigDescriptor.

As we know ConfigDescriptor represents a program that describes the retrieval of config parameters. In fact, the same program can be used to write back the config in various shapes.

Either case, a ConfigDescriptor can exist without a Source attached.

Example:

val stringConfig: ConfigDescriptor[String] = string("USERNAME")

Later on we can read the config by attaching a source.

val result = read(stringConfig from ConfigSource.fromMap(Map.empty))

However, you can attach a source to the configDescriptor at an earlier stage.

For example:

val stringConfig: ConfigDescriptor[String] =
  string("USERNAME") from ConfigSource.fromMap(Map.empty)

Later on, you can simply read it using:

val result = read(stringConfig)

Using unsourced, you can now untag the source from stringConfig.

val stringConfigNoSource: ConfigDescriptor[String] =
  stringConfig.unsourced

This can be useful in test cases where you want to remove a source and attach a different source.

Example:

val testSource: ConfigSource = ConfigSource.fromMap(Map(..))

val result = stringConfig.unsourced from testSource

updateSource can update the source of an existing ConfigDescriptor

Example:

val configSource1 = ConfigSource.fromMap(Map.empty)
val configSource2 = ConfigSource.fromMap(Map("USERNAME" -> "abc"))

val config = string("USERNAME") from configSource1

val updatedConfig = config updateSource (_ orElse configSource2)

In the above example, we update the existing ConfigDescriptor to try another ConfigSource called configSource2, if it fails to retrieve the value of USERNAME from configSource1.

zip is a ConfigDescriptor builder. We know ConfigDescriptor is a program that describes the retrieval of a set of configuration parameters.

Below given is a ConfigDescriptor that describes the retrieval of a single config.

val port: ConfigDescriptor[String] = string("PORT")

However, in order to retrieve multiple configuration parameters, we can make use of zip.

Example:

final case class Config(userName: String, port: Int)

object Config {
   val dbConfig: ConfigDescriptor[Config] =
      (string("USERNAME") zip int("PORT")).to[Config]
}