On May 19, 2014, at 1:44 PM, Ben Wolfson <wolf...@gmail.com> wrote: > I wouldn't say that I *often* find myself reaching for monads, or the state > monad in particular, but I certainly have found them useful on occasion (and > would have sometimes refrained from using them where I'd naturally lean to > doing so solely to avoid creating an dependency). For instance, whenever > there's a couple of functions that return either a success value or an error > message that have to be threaded together---an error monad to do the plumbing > makes this a lot nicer.
I badly miss the Maybe and Either monads, but would want the syntactic support Haskell provides (which I can't see will ever be available in Clojure) > > We've got a library at ReadyForZero for walking though json and xml > structures and doing transformations based on the values found there, or a > bit of configuration, using a combined reader-writer-state monad, and a bunch > of code that uses it. The state that's held is actually just a zipper into > the structure, the configuration at this point is only a keyword, and the > writer log holds reports of unexpected values. This could all be done with > other machinery---pass the zipper around directly (or hold it in an atom), > pass the log around directly (or hold it in an atom), use a dynamic variable > + binding for the configuration (since the reader monad amounts to that > anyway). However, I think the monadic code is easier to work with, partly > because nothing does need to be managed or passed around explicitly (so it's > easier to put together lots of little pieces), and partly because it enables > the use of generic tools. Also, traversing the the structures has a fairly > imperative feel---go here, go there, do this transformation---with occasional > variable binding, and the macro for monadic computations the monad library > we're using provides makes expressing that fairly convenient. (Though I may > be biased, since I wrote it.) > > It's true that there doesn't seem to be much need for introducing a monad > library and using the state monad if you *only* had the state monad, since > Clojure has other ways to deal with mutation (incidentally, I don't think > it's true to say that Haskell only has refs, not atoms; there are functions > to modify IORefs, which live outside the STM system, atomically). Which is why I didn't call them atoms :-) There's also a ref in the ST monad (which is a bells-and-whistles state monad that has fallen into a bit of disuse since the IO monad appeared) > > > > On Fri, May 16, 2014 at 5:49 PM, Julian <juliangam...@gmail.com> wrote: > A quick shoutout to the Clojure Community - thanks for the way you've all > contributed to make my life (mentally) richer. > > James Reeves (author of Compojure and many other wonderful libraries) made > this interesting comment on Hacker News: > > Clojure has libraries that implement monads, but these aren't often used > > for threading state. I can't quite place my finger on why, but in Clojure I > > rarely find myself reaching for something like the state monad, as I would > > in Haskell. > >Clojure tends to view mutability as a concurrency problem, and the tools it > >provides to deal with mutability, such as atoms, refs, agents, channels and > >so forth, are not mechanisms to avoid mutation, as to provide various > >guarantees that restrict it in some fashion. > >It might be that in the cases where I'd use a state monad in Haskell, in > >Clojure I might instead use an atom. They're in no way equivalent, but they > >have some overlapping use-cases. > https://news.ycombinator.com/item?id=7751424 > My question is - have other Clojure/Haskell programmers had this experience? > (ie "I rarely find myself reaching for something like the state monad"). I'm > interested to hear if so, and why. > JG > PS If this post is unhelpful, could be worded better - please let me know. > I'm asking out of curiosity, not with intent to troll. > > -- > 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/d/optout. > > > > -- > 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] > > > -- > 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/d/optout. -- 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/d/optout.
