I don't know if this is adequately "in the wild" (since it's part of a monad lib itself), but part of my monads library uses a continuation monad to let two mutually recursive functions traverse and rebuild a tree structure without blowing the stack. It would be a pain to do this kind of thing by hand, since (unlike in the typical even/odd example where the built-in trampoline function is simple to use) the recursive calls aren't naturally written as tail calls. The use of a continuation monad effectively results in the functions' being rewritten in CPS, and the implementation does the trampolining for you:
https://github.com/bwo/monads/blob/master/src/monads/cont.clj#L62 ---------------------------------------------------------------------- (declare reorg-binds) (defn- reorg-plus [m] (if-instance Mplus m (let [l (.l m) r (.r m)] (mdo li <- (reorg-binds l) ri <- (reorg-binds r) ri <- (if-instance Mplus ri (let [lr (.l ri)] (if-instance Mplus lr (reorg-plus ri) (return ri))) (return ri)) (if-instance Mplus li (let [l-l (.l li)] (mdo ri <- (reorg-plus ri) (reorg-plus (Mplus. l-l (Mplus. (.r li) ri))))) (return (Mplus. li ri))))) (return m))) (defn- reorg-binds [m] (if-instance Bind m (let [comp (.comp m)] (if-instance Bind comp (let [i-comp (.comp comp) i-f (.f comp) f (.f m)] (mdo i <- (reorg-plus i-comp) (reorg-binds (Bind. i (fn [x] (Bind. (i-f x) f)))))) (mdo comp <- (reorg-plus comp) (return (Bind. comp (.f m)))))) (return m))) (defn reorganize "Reorganize the monadic computation m so that binds nested on the left are moved to the right, i.e. transform expressions like (>>= (>>= (>>= m f) g) h) into expressions like (>>= m (fn [x] (>>= (f x) (fn [y] (>>= (g y) h))))). A monad implementation for which these two expressions give different results is broken. Similarly reorganizes mplus operations nested on the left: (mplus (mplus (mplus a b) c) d) Becomes (mplus a (mplus b (mplus c d))). And, similarly, an mplus operation which is not associative is considered to be in error. Both transformations are interleaved: (mplus (mplus (>>= (>>= (mplus (mplus a b) c) f) g) d) e) becomes (mplus (>>= (mplus a (mplus b c)) (fn [x] (>>= (f x) g))) (mplus d e)). Note that *only* mplus and bind operations are investigated: the reorganization does not recurse into the monadic-computation arguments of e.g. listen, local, or pass." [m] ;; this reorg-plus is here in case we fall through reorg-binds right ;; away. We can't call reorg-plus in reorg-binds in the else branch ;; of the first if-instance, because it can lead to, yes, stack ;; overflows. (run-cont (reorg-plus (run-cont (reorg-binds m))))) ---------------------------------------------------------------------- On Mon, Apr 8, 2013 at 7:56 AM, Carlos Galdino <carloshsgald...@gmail.com>wrote: > Hi, > > I've been reading about monads for the past couple of weeks and I think I > got the idea, but I still don't know when to use it since I don't have > enough experience with functional programming. > > So, I'd like to ask you guys if you can point me to some examples of > Monads usage "in the wild". Because I've seen a lot of simple examples but > not a real one, used in a library, etc. > > Does anyone know a good example of real world usage? > > Thanks in advance. > > -- > -- > 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. > > > -- 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/groups/opt_out.
