It forces the go block to wait until (walker (:left t)) pushes something -- that is, for the recursively-entered go block to complete. Similarly the right branch is waited for before the parent go block completes. So forcing an in-order traversal of the tree.
On Fri, Sep 13, 2013 at 5:13 PM, larry google groups < lawrencecloj...@gmail.com> wrote: > > I am stupid and recursion is clearly beyond my intellect. Martin Trojer > has a great blog post here which I learned a lot from but I don't > understand why the final example works: > > > http://martintrojer.github.io/clojure/2013/07/17/non-tailrecursive-functions-in-coreasync/ > > He offers this as an example of recursively walking a binary search tree: > > (defn walk [tree ch] > (letfn [(walker [t] > (go > (when t > (walker (:left t)) > (>! ch (:value t)) > (walker (:right t)))))] > (go > (<! (walker tree)) > (close! ch)))) > > and then he writes: > > "This looks promising, but the results in the channel can be in any order > (since there are no order guarantees in the scheduling of go processes) – > this also means that some of the values might be missing since the “top” go > process can be scheduled before a child one. We need a little bit more > synchronisation to arrive at a working solution." > > > and then offers this as the final working example: > > (defn walk [tree ch] > (letfn [(walker [t] > (go > (when t > (<! (walker (:left t))) > (>! ch (:value t)) > (<! (walker (:right t))))))] > (go > (<! (walker tree)) > (close! ch)))) > > I am confused what this line does: > > (<! (walker (:left t))) > > It looks like we are pulling a value off channel that's returned from that > call to walker? But why? We seem to be throwing this value away? I don't see > it being stored anywhere. > > How does this give us synchronization? > > -- > -- > 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.
