Evidence type A is not equal to type B.

Based on https://github.com/milessabin/shapeless.

A simple, macro-less means of creating accessors from Services. Extend the companion object with Accessible[ServiceName], then simply call Companion(_.someMethod), to return a ZIO effect that requires the Service in its environment.

Example:

trait FooService {
  def magicNumber: UIO[Int]
  def castSpell(chant: String): UIO[Boolean]
}

object FooService extends Accessible[FooService]

val example: ZIO[Has[FooService], Nothing, Unit] =
  for {
    int  <- FooService(_.magicNumber)
    bool <- FooService(_.castSpell("Oogabooga!"))
  } yield ()

The entry point for a purely-functional application on the JVM.

import zio.App
import zio.Console._

object MyApp extends App {

  final def run(args: List[String]) =
    myAppLogic.exitCode

  val myAppLogic =
    for {
      _ <- printLine("Hello! What is your name?")
      n <- readLine
      _ <- printLine("Hello, " + n + ", good to meet you!")
    } yield ()
}

A Chunk[A] represents a chunk of values of type A. Chunks are designed are usually backed by arrays, but expose a purely functional, safe interface to the underlying elements, and they become lazy on operations that would be costly with arrays, such as repeated concatenation.

The implementation of balanced concatenation is based on the one for Conc-Trees in "Conc-Trees for Functional and Parallel Programming" by Aleksandar Prokopec and Martin Odersky. http://aleksandar-prokopec.com/resources/docs/lcpc-conc-trees.pdf

NOTE: For performance reasons Chunk does not box primitive types. As a result, it is not safe to construct chunks from heterogeneous primitive types.

A ChunkBuilder[A] can build a Chunk[A] given elements of type A. ChunkBuilder is a mutable data structure that is implemented to efficiently build chunks of unboxed primitives and for compatibility with the Scala collection library.

ChunkLike represents the capability for a Chunk to extend Scala's collection library. Because of changes to Scala's collection library in 2.13, separate versions of this trait are implemented for 2.11 / 2.12 and 2.13 / Dotty. This allows code in Chunk to be written without concern for the implementation details of Scala's collection library to the maximum extent possible.

Note that IndexedSeq is not a referentially transparent interface in that it exposes methods that are partial (e.g. apply), allocate mutable state (e.g. iterator), or are purely side effecting (e.g. foreach). Chunk extends IndexedSeq to provide interoperability with Scala's collection library but users should avoid these methods whenever possible.

Describes a strategy for evaluating multiple effects, potentially in parallel. There are three possible execution strategies: Sequential, Parallel, and ParallelN.

An Exit[E, A] describes the result of executing an IO value. The result is either succeeded with a value A, or failed with a Cause[E].

A fiber is a lightweight thread of execution that never consumes more than a whole thread (but may consume much less, depending on contention and asynchronicity). Fibers are spawned by forking ZIO effects, which run concurrently with the parent effect.

Fibers can be joined, yielding their result to other fibers, or interrupted, which terminates the fiber, safely releasing all resources.

def parallel[A, B](io1: Task[A], io2: Task[B]): Task[(A, B)] =
  for {
    fiber1 <- io1.fork
    fiber2 <- io2.fork
    a      <- fiber1.join
    b      <- fiber2.join
  } yield (a, b)

Represents a failure in a fiber. This could be caused by some non- recoverable error, such as a defect or system error, by some typed error, or by interruption (or combinations of all of the above).

This class is used to wrap ZIO failures into something that can be thrown, to better integrate with Scala exception handling.

The trait Has[A] is used with ZIO environment to express an effect's dependency on a service of type A. For example, RIO[Has[Console], Unit] is an effect that requires a Console service.

Services parameterized on path dependent types are not supported.

The InterruptStatus of a fiber determines whether or not it can be interrupted. The status can change over time in different regions.

A NonEmptyChunk is a Chunk that is guaranteed to contain at least one element. As a result, operations which would not be safe when performed on Chunk, such as head or reduce, are safe when performed on NonEmptyChunk. Operations on NonEmptyChunk which could potentially return an empty chunk will return a Chunk instead.

A promise represents an asynchronous variable, of zio.IO type, that can be set exactly once, with the ability for an arbitrary number of fibers to suspend (by calling await) and automatically resume when the variable is set.

Promises can be used for building primitive actions whose completions require the coordinated action of multiple fibers, and for building higher-level concurrent or asynchronous structures.

for {
  promise <- Promise.make[Nothing, Int]
  _       <- promise.succeed(42).delay(1.second).fork
  value   <- promise.await // Resumes when forked fiber completes promise
} yield value

A Reservation[-R, +E, +A] encapsulates resource acquisition and disposal without specifying when or how that resource might be used.

See ZManaged#reserve and ZIO#reserve for details of usage.

The entry point for a purely-functional application on the JVM.

import zio.ZApp
import zio.Console._

object MyApp extends ZApp[Has[Console]] {

  def environment: Has[Console] = Has(ConsoleLive)

  final def run(args: List[String]) =
    myAppLogic.exitCode

  def myAppLogic =
    for {
      _ <- printLine("Hello! What is your name?")
      n <- readLine
      _ <- printLine("Hello, " + n + ", good to meet you!")
    } yield ()
}

A FiberRef is ZIO's equivalent of Java's ThreadLocal. The value of a FiberRef is automatically propagated to child fibers when they are forked and merged back in to the value of the parent fiber after they are joined.

for {
  fiberRef <- FiberRef.make("Hello world!")
  child    <- fiberRef.set("Hi!).fork
  result   <- child.join
} yield result

Here result will be equal to "Hi!" since changed made by a child fiber are merged back in to the value of the parent fiber on join.

By default the value of the child fiber will replace the value of the parent fiber on join but you can specify your own logic for how values should be merged.

for {
  fiberRef <- FiberRef.make(0, math.max)
  child    <- fiberRef.update(_ + 1).fork
  _        <- fiberRef.update(_ + 2)
  _        <- child.join
  value    <- fiberRef.get
} yield value

Here value will be 2 as the value in the joined fiber is lower and we specified max as our combining function.

A ZHub[RA, RB, EA, EB, A, B] is an asynchronous message hub. Publishers can publish messages of type A to the hub and subscribers can subscribe to take messages of type B from the hub. Publishing messages can require an environment of type RA and fail with an error of type EA. Taking messages can require an environment of type RB and fail with an error of type EB.

A ZIO[R, E, A] value is an immutable value that lazily describes a workflow or job. The workflow requires some environment R, and may fail with an error of type E, or succeed with a value of type A.

These lazy workflows, referred to as _effects_, can be informally thought of as functions in the form:

R => Either[E, A]

ZIO effects model resourceful interaction with the outside world, including synchronous, asynchronous, concurrent, and parallel interaction.

ZIO effects use a fiber-based concurrency model, with built-in support for scheduling, fine-grained interruption, structured concurrency, and high scalability.

To run an effect, you need a Runtime, which is capable of executing effects. Runtimes bundle a thread pool together with the environment that effects need.

A ZLayer[A, E, B] describes a layer of an application: every layer in an application requires some services (the input) and produces some services (the output).

Layers can be thought of as recipes for producing bundles of services, given their dependencies (other services).

Construction of layers can be effectful and utilize resources that must be acquired and safely released when the services are done being utilized.

By default layers are shared, meaning that if the same layer is used twice the layer will only be allocated a single time.

Because of their excellent composition properties, layers are the idiomatic way in ZIO to create services that depend on other services.

A ZManaged[R, E, A] is a managed resource of type A, which may be used by invoking the use method of the resource. The resource will be automatically acquired before the resource is used, and automatically released after the resource is used.

Resources do not survive the scope of use, meaning that if you attempt to capture the resource, leak it from use, and then use it after the resource has been consumed, the resource will not be valid anymore and may fail with some checked error, as per the type of the functions provided by the resource.

A ZQueue[RA, RB, EA, EB, A, B] is a lightweight, asynchronous queue into which values of type A can be enqueued and of which elements of type B can be dequeued. The queue's enqueueing operations may utilize an environment of type RA and may fail with errors of type EA. The dequeueing operations may utilize an environment of type RB and may fail with errors of type EB.

A ZRef[RA, RB, EA, EB, A, B] is a polymorphic, purely functional description of a mutable reference. The fundamental operations of a ZRef are set and get. set takes a value of type A and sets the reference to a new value, requiring an environment of type RA and potentially failing with an error of type EA. get gets the current value of the reference and returns a value of type B, requiring an environment of type RB and potentially failing with an error of type EB.

When the error and value types of the ZRef are unified, that is, it is a ZRef[R, R, E, E, A, A], the ZRef also supports atomic modify and update operations. All operations are guaranteed to be safe for concurrent access.

By default, ZRef is implemented in terms of compare and swap operations for maximum performance and does not support performing effects within update operations. If you need to perform effects within update operations you can create a ZRef.Synchronized, a specialized type of ZRef that supports performing effects within update operations at some cost to performance. In this case writes will semantically block other writers, while multiple readers can read simultaneously.

ZRef.Synchronized also supports composing multiple ZRef.Synchronized values together to form a single ZRef.Synchronized value that can be atomically updated using the zip operator. In this case reads and writes will semantically block other readers and writers.

NOTE: While ZRef provides the functional equivalent of a mutable reference, the value inside the ZRef should normally be immutable since compare and swap operations are not safe for mutable values that do not support concurrent access. If you do need to use a mutable value ZRef.Synchronized will guarantee that access to the value is properly synchronized.

A ZScope[A] is a value that allows adding finalizers identified by a key. Scopes are closed with a value of type A, which is provided to all the finalizers when the scope is released.

For safety reasons, this interface has no method to close a scope. Rather, an open scope may be required with ZScope.make, which returns a function that can close a scope. This allows scopes to be safely passed around without fear they will be accidentally closed.

ZState[S] models a value of type S that can be read from and written to during the execution of an effect. The idiomatic way to work with ZState is as part of the environment using operators defined on ZIO. For example:

final case class MyState(counter: Int)

for {
  _     <- ZIO.updateState[MyState](state => state.copy(counter = state.counter + 1))
  count <- ZIO.getStateWith[MyState](_.counter)
} yield count

Because ZState is typically used as part of the environment, it is recommended to define your own state type S such as MyState above rather than using a type such as Int to avoid the risk of ambiguity.

To run an effect that depends on some state, create the initial state with the make constructor and then use toLayer to convert it into a layer that you can provide along with your application's other dependencies.