github.com/lukechampine/stm (go): Package stm provides Software Transactional Memory operations for Go. This is an alternative to the standard way of writing concurrent code (channels and mutexes). STM makes it easy to perform arbitrarily complex operations in an atomic fashion. One of its primary advantages over traditional locking is that STM transactions are composable, whereas locking functions are not -- the composition will either deadlock or release the lock between functions (making it non-atomic).  To begin, create an STM object that wraps the data you want to access concurrently.  You can then use the Atomically method to atomically read and/or write the the data. This code atomically decrements x:  An important part of STM transactions is retrying. At any point during the transaction, you can call tx.Retry(), which will abort the transaction, but not cancel it entirely. The call to Atomically will block until another call to Atomically finishes, at which point the transaction will be rerun. Specifically, one of the values read by the transaction (via tx.Get) must be updated before the transaction will be rerun. As an example, this code will try to decrement x, but will block as long as x is zero:  Internally, tx.Retry simply calls panic(stm.Retry). Panicking with any other value will cancel the transaction; no values will be changed. However, it is the responsibility of the caller to catch such panics.  Multiple transactions can be composed using Select. If the first transaction calls Retry, the next transaction will be run, and so on. If all of the transactions call Retry, the call will block and the entire selection will be retried. For example, this code implements the "decrement-if-nonzero" transaction above, but for two values. It will first try to decrement x, then y, and block if both values are zero.  An important caveat: transactions must be idempotent (they should have the same effect every time they are invoked). This is because a transaction may be retried several times before successfully completing, meaning its side effects may execute more than once. This will almost certainly cause incorrect behavior. One common way to get around this is to build up a list of impure operations inside the transaction, and then perform them after the transaction completes.  The stm API tries to mimic that of Haskell's Control.Concurrent.STM, but this is not entirely possible due to Go's type system; we are forced to use interface{} and type assertions. Furthermore, Haskell can enforce at compile time that STM variables are not modified outside the STM monad. This is not possible in Go, so be especially careful when using pointers in your STM code. Remember: modifying a pointer is a side effect!
Keywords: stm