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.