On Tue, Feb 19, 2013 at 2:28 AM, Philip Potter
<philip.g.pot...@gmail.com> wrote:
>
> They sound like very interesting features. They also sound lower-level
> than Clojure's STM -- if a Clojure STM transaction fails, it will
> automatically retry until it succeeds (or hits
> LockingTransaction/RETRY_LIMIT, currently 10000, when it throws an
> exception.) Is this the case in Haskell?
I don't know if there's a limit, but transactions retry automatically,
and transaction failure is detected automatically. I'm confused by the
characterization of orElse as lower-level than Clojure's STM: it's a
way of combining two independent STM actions and thus strikes me as a
higher-level operation.
Here's the way "retry" is introduced in "Beautiful Concurrency":
"""
Atomic blocks as we have introduced them so far are utterly inadequate to
coordinate concurrent programs. They lack two key facilities: blocking and
choice. In this section I’ll describe how the basic STM interface is elaborated
to include them in a fully-modular way.
Suppose that a thread should block if it attempts to overdraw an account (i.e.
withdraw more than the current balance). Situations like this are common in
concurrent programs: for example, a thread should block if it reads from an
empty buffer, or when it waits for an event. We achieve this in STM by adding
the single function retry, whose type is
retry :: STM a
Here is a modified version of withdraw that blocks if the balance would go
negative:
limitedWithdraw :: Account -> Int -> STM ()
limitedWithdraw acc amount
= do { bal <- readTVar acc
; if amount > 0 && amount > bal
then retry
else writeTVar acc (bal - amount) }
"""
So it isn't really about restarting failed transactions because (e.g.)
someone else has written to the ref while we've been executing, but
detecting a case within an otherwise successful transaction---where
"successful" means "we can read and write to all these refs with no
problem"---where we actually want to start over and wait until some
values have changed.
--
Ben Wolfson
"Human kind has used its intelligence to vary the flavour of drinks,
which may be sweet, aromatic, fermented or spirit-based. ... Family
and social life also offer numerous other occasions to consume drinks
for pleasure." [Larousse, "Drink" entry]
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