v0.10.0-M1
This is a big release! (In fact, it's basically a from-scratch rewrite). Unfortunately it involves some source-incompatible changes but hopefully you'll agree the benefits are very much worth it! Highlights include:
- 'Honest effect polymorphism': previously we could run
STM.atomically[IO](txn1)andSTM.atomically[OptionT[IO, *]](txn2)and run the risk of class-cast exceptions as we tracked state internally that was parameterized onFwithout that being tracked in the type system. Now the design looks like:
trait STM[F[_]] {
class TVar[A] {
def get: Txn[A]
}
trait Txn[A]
def commit[A](txn: Txn[A]): F[A]
}which means that a Txn is tied to the effect type of the STM instance which created it and hence safe
- Stack-safe construction and evaluation of
Txns - Fine-grained locking: when committing we only acquire a lock on the
TVars that we are comitting, rather than a global lock, and hence can support a higher degree of parallelism - Fully semantic-blocking: one of the worst features of previous versions was that it would acquire a global lock via a blocking
STM.synchronized- something you should never do on anIOthread. Now the implementation is entirely based on the Cats Effect semantically-blockingRefandSemaphoreabstractions - a
MonadError[Txn, Throwable]instance which enables backtracking in a transaction eg
for {
tvar <- stm.commit(TVar.of(0))
_ <- stm.commit(
for {
_ <- tvar.set(1)
_ <- (tvar.modify(_ + 1) >> stm.abort(new RuntimeException("BOOM"))).handleErrorWith {
case _ => tvar.modify(_ + 2)
}
} yield ()
)
v <- stm.commit(tvar.get)
res <- IO(assertEquals(v, 3))
} yield res- Laws governing the interaction of the
stmcombinators - Support for Cats Effect 3! Unfortunately this release actually depends on some specifics of cats effect 3 so it has not (yet at least) been back-ported to cats effect 2. The expectation is that the non-milestone release will happen as soon as cats effect 3 is released
- Defining a lot of derived cats combinators directly on
Txnfor convenience and discoverability in IDEs