On 18 February 2013 21:41, Tom Hall <thattommyh...@gmail.com> wrote: >>> Why does Clojure not need retry or orElse when another implementer of >>> STM considers them essential? >> What are retry and orElse? What do they do? > > I had hoped to get a reply from someone with experience of both, as > the quote suggests they are for blocking and choice (The article was > the first time I'd seen STM so am absolutely not an expert) > > From the Haskell docs: > retry :: STM a > Retry execution of the current memory transaction because it has seen > values in TVars which mean that it should not continue (e.g. the TVars > represent a shared buffer that is now empty). The implementation may > block the thread until one of the TVars that it has read from has been > udpated. (GHC only) > (this is what made me think of watchers in clojure, > http://clojure.github.com/clojure/clojure.core-api.html#clojure.core/add-watch) > > orElse :: STM a -> STM a -> STM a > Compose two alternative STM actions (GHC only). If the first action > completes without retrying then it forms the result of the orElse. > Otherwise, if the first action retries, then the second action is > tried in its place. If both actions retry then the orElse as a whole > retries.
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? Clojure automatically detects the conditions under which a transaction is considered to have failed (modulo the need to avoid write skew with the #'ensure fn) -- does Haskell provide this or are you expected to do all the checking yourself and call retry directly? Does Haskell provide a way to easily achieve Clojure's auto-retrying behaviour? Given the behaviour, I don't think it would be possible to implement retry or orElse without significant modifications to LockingTransaction.java and possibly exposing a different set of abstractions as STM primitives -- the retrying behaviour is written into the java source: https://github.com/clojure/clojure/blob/master/src/jvm/clojure/lang/LockingTransaction.java#L252 >> "Santa repeatedly sleeps until wakened by either all of his nine >> reindeer, back from their holidays, or by a group of three of his ten >> elves. If awakened by the reindeer, he harnesses each of them to his >> sleigh, delivers toys with them and finally unharnesses them (allowing >> them to go off on holiday). If awakened by a group of elves, he shows >> each of the group into his study, consults with them on toy R&D and >> finally shows them each out (allowing them to go back to work). Santa >> should give priority to the reindeer in the case that there is both a >> group of elves and a group of reindeer waiting." > > I think it's a shame he wrote the article around Christmas and chose a > less well known problem, I have looked at several Clojure Barbershop > Problem solutions so have a better idea now what an idiomatic solution > to this sort of problem might be. > > Cheers Phil, > Tom > > -- > -- > You received this message because you are subscribed to the Google > Groups "Clojure" group. > To post to this group, send email to clojure@googlegroups.com > Note that posts from new members are moderated - please be patient with your > first post. > To unsubscribe from this group, send email to > clojure+unsubscr...@googlegroups.com > For more options, visit this group at > http://groups.google.com/group/clojure?hl=en > --- > You received this message because you are subscribed to the Google Groups > "Clojure" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to clojure+unsubscr...@googlegroups.com. > For more options, visit https://groups.google.com/groups/opt_out. > > -- -- You received this message because you are subscribed to the Google Groups "Clojure" group. To post to this group, send email to clojure@googlegroups.com Note that posts from new members are moderated - please be patient with your first post. To unsubscribe from this group, send email to clojure+unsubscr...@googlegroups.com For more options, visit this group at http://groups.google.com/group/clojure?hl=en --- You received this message because you are subscribed to the Google Groups "Clojure" group. To unsubscribe from this group and stop receiving emails from it, send an email to clojure+unsubscr...@googlegroups.com. For more options, visit https://groups.google.com/groups/opt_out.
