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 a variable if it exists. Implemented in the successor MVar2
• swap takes a value, replaces it and returns the taken value. Implemented in the successor MVar2
• 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 doesn't block any threads, all waiting being done asynchronously (via Async or Concurrent data types, such as IO).

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

Inspired by Control.Concurrent.MVar from Haskell and by scalaz.concurrent.MVar.

The MVar2 is the successor of MVar with tryRead and swap. It was implemented separately only to maintain binary compatibility with MVar.