Packages

final class MVar[F[_], A] extends MVar2[F, A]

A mutable location, that is either empty or contains a value of type A.

It has the following fundamental atomic operations:

  • put which fills the var if empty, or blocks (asynchronously) until the var is empty again
  • tryPut which fills the var if empty. returns true if successful
  • take which empties the var if full, returning the contained value, or blocks (asynchronously) otherwise until there is a value to pull
  • tryTake empties if full, returns None if empty.
  • read which reads the current value without touching it, assuming there is one, or otherwise it waits until a value is made available via put
  • tryRead returns Some(a) if full, without modifying the var, or else returns None
  • isEmpty returns true if currently empty

The MVar is appropriate for building synchronization primitives and performing simple inter-thread communications. If it helps, it's similar with a BlockingQueue(capacity = 1), except that it is pure and that doesn't block any threads, all waiting being done asynchronously.

Given its asynchronous, non-blocking nature, it can be used on top of Javascript as well.

N.B. this is a reimplementation of the interface exposed in Cats-Effect, see: cats.effect.concurrent.MVar

Inspired by Control.Concurrent.MVar from Haskell.

Source
MVar.scala
Linear Supertypes
MVar2[F, A], cats.effect.concurrent.MVar[F, A], MVarDocumentation, AnyRef, Any
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Visibility
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Value Members

  1. def imapK[G[_]](f: ~>[F, G], g: ~>[G, F]): MVar2[G, A]
    Definition Classes
    MVar2
  2. def isEmpty: F[Boolean]

    Returns true if the var is empty, false if full.

    Returns true if the var is empty, false if full.

    Definition Classes
    MVar → MVar
  3. def modify[B](f: (A) => F[(A, B)]): F[B]

    Modifies the contents of the MVar using the effectful function f, but also allows for returning a value derived from the original contents of the MVar.

    Modifies the contents of the MVar using the effectful function f, but also allows for returning a value derived from the original contents of the MVar. Like use, in case of failure, it sets the contents of the MVar to the original value.

    f

    effectful function that operates on the contents of this MVar

    returns

    the second value produced by applying f to the contents of this MVar

    Definition Classes
    MVar → MVar2
    Note

    This operation is only safe from deadlocks if there are no other producers for this MVar.

  4. def modify_(f: (A) => F[A]): F[Unit]

    Modifies the contents of the MVar using the effectful function f.

    Modifies the contents of the MVar using the effectful function f. Like use, in case of failure, it sets the contents of the MVar to the original value.

    f

    effectful function that operates on the contents of this MVar

    returns

    no useful value. Executed only for the effects.

    Definition Classes
    MVar → MVar2
    Note

    This operation is only safe from deadlocks if there are no other producers for this MVar.

  5. def put(a: A): F[Unit]

    Fills the MVar if it is empty, or blocks (asynchronously) if the MVar is full, until the given value is next in line to be consumed on take.

    Fills the MVar if it is empty, or blocks (asynchronously) if the MVar is full, until the given value is next in line to be consumed on take.

    This operation is atomic.

    returns

    a task that on evaluation will complete when the put operation succeeds in filling the MVar, with the given value being next in line to be consumed

    Definition Classes
    MVar → MVar
  6. def read: F[A]

    Tries reading the current value, or blocks (asynchronously) until there is a value available.

    Tries reading the current value, or blocks (asynchronously) until there is a value available.

    This operation is atomic.

    returns

    a task that on evaluation will be completed after a value has been read

    Definition Classes
    MVar → MVar
  7. def swap(newValue: A): F[A]

    Replaces a value in MVar and returns the old value.

    Replaces a value in MVar and returns the old value.

    newValue

    is a new value

    returns

    the value taken

    Definition Classes
    MVar → MVar2
    Note

    This operation is only safe from deadlocks if there are no other producers for this MVar.

  8. def take: F[A]

    Empties the MVar if full, returning the contained value, or blocks (asynchronously) until a value is available.

    Empties the MVar if full, returning the contained value, or blocks (asynchronously) until a value is available.

    This operation is atomic.

    returns

    a task that on evaluation will be completed after a value was retrieved

    Definition Classes
    MVar → MVar
  9. def tryPut(a: A): F[Boolean]

    Fill the MVar if we can do it without blocking,

    Fill the MVar if we can do it without blocking,

    returns

    whether or not the put succeeded

    Definition Classes
    MVar → MVar
  10. def tryRead: F[Option[A]]

    Tries reading the current value, returning Some(a) if the var is full, but without modifying the var in any way.

    Tries reading the current value, returning Some(a) if the var is full, but without modifying the var in any way. Or None if the var is empty.

    Definition Classes
    MVar → MVar2
  11. def tryTake: F[Option[A]]

    Empty the MVar if full

    Empty the MVar if full

    returns

    an Option holding the current value, None means it was empty

    Definition Classes
    MVar → MVar
  12. def use[B](f: (A) => F[B]): F[B]

    Applies the effectful function f on the contents of this MVar.

    Applies the effectful function f on the contents of this MVar. In case of failure, it sets the contents of the MVar to the original value.

    f

    effectful function that operates on the contents of this MVar

    returns

    the value produced by applying f to the contents of this MVar

    Definition Classes
    MVar → MVar2
    Note

    This operation is only safe from deadlocks if there are no other producers for this MVar.

Deprecated Value Members

  1. def mapK[G[_]](f: ~>[F, G]): cats.effect.concurrent.MVar[G, A]
    Definition Classes
    MVar
    Annotations
    @deprecated
    Deprecated

    (Since version 2.2.0) mapK is deprecated in favor of imapK which supports the new invariant MVar2 interface