On 27 August 2013 20:45, Timothy Baldridge <tbaldri...@gmail.com> wrote:
> The reason for not allowing nils isn't a complex one, and basically boils > down to the following: > > a) to avoid race conditions, we need a single value to signal "the channel > is closed". As mentioned, nil is the obvious choice for this as it matches > lazy seqs and fits well with the rest of clojure: > > Agreed that you want a single sentinel value. It doesn't match lazy-seqs at all though: lazy seqs can contain nils just fine. There's a big difference between (next some-lazy-seq) [which could be nil, indicating an empty sequence] and the actual values in the seq [which could also be nil but don't indicate the end of the seq]. > (when-let [v (<! c)] > (process v)) > > If we chose a different value, this becomes much more ugly: > > (let [v (<! c)] > (when-not (= v :async/closed) > (process v))) > > This can be solved easily by providing a macro or some other predicate that knows how to check for the sentinel value correctly. e.g. (when-more [v (<! c)] (process v)) > b) I question if there are any valid uses for putting nil in a channel. > With all due respect to all who have written here, thus far, every > complaint about nils and channels boils down to a conversion from seqs to > channels. This is the wrong way to look at the problem. Channels are > co-ordination primitives not data structures. Simply because a lazy seq > looks like a channel, doesn't mean that they should be treated as such. > > In all the core.async code I've written I've never had to put a nil in a > channel, so I'm left with the uncomfortable conclusion that most complaints > on this subject are contrived. I could be wrong, but I just haven't seen a > valid use case yet. > > To me it's all about consistency with other Clojure constructs. You can safely put nils in sequences, vectors, lists, sets etc.. nil is a valid "value" just like anything else. So why can't you put them in a channel? Two use cases I have encountered that motivate this: a) what if you want to send a sequence through a channel? Since nil as a value represents the empty sequence, you have to put in some extra special case handling with the current core.async model. b) what if you want to write generic code to send all the values in an arbitrary collection through a channel? you would have to wrap/unwrap nils at either end to make this work currently. Both of these, I think, are reasonable and common enough use cases that it's worth supporting them elegantly rather than forcing users to implement their own nil-wrapping functionality. > This all being said, there really isn't a technical reason to not allow > nils, it just simplifies much of the design and that probably translates to > better performance. So the restriction could be lifted if a rock solid > reason could be found, but as of yet, I haven't seen it. > I don't believe there is any noticeable performance difference between checking for nil and checking if a value is identical? to some sentinel value (which would presumably be static, final, immutable and hence very well optimised by the JVM). In addition, not allowing nils just means you have to do extra work to wrap/unwrap nils as a user - which is almost certainly a net loss on overall performance. Still, I think consistency is more significant than the performance argument in this case. > > Timothy Baldridge > > > On Tue, Aug 27, 2013 at 2:12 AM, Max Penet <m...@qbits.cc> wrote: > >> It's a real problem for me too, I also wonder what was the intention >> behind this. I guess there could be a very good reason for this special >> treatement of nils, but I haven't seen it yet. >> >> I would love to hear about this from people involved in core.async >> development. >> >> On Friday, August 16, 2013 4:44:48 AM UTC+2, Mikera wrote: >>> >>> Hi all, >>> >>> I'm experimenting with core.async. Most of it is exceptionally good, but >>> bit I'm finding it *very* inconvenient that nil can't be sent over >>> channels. In particular, you can't pipe arbitrary Clojure sequences through >>> channels (since sequences can contain nils). >>> >>> I see this as a pretty big design flaw given the ubiquity of sequences >>> in Clojure code - it appears to imply that you can't easily compose >>> channels with generic sequence-handling code without some pretty ugly >>> special-case handling. >>> >>> Am I missing something? Is this a real problem for others too? >>> >>> If it is a design flaw, can it be fixed before the API gets locked down? >>> >> -- >> -- >> 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. >> > > > > -- > “One of the main causes of the fall of the Roman Empire was that–lacking > zero–they had no way to indicate successful termination of their C > programs.” > (Robert Firth) > > -- > -- > 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 a topic in the > Google Groups "Clojure" group. > To unsubscribe from this topic, visit > https://groups.google.com/d/topic/clojure/pF9FEP7b77U/unsubscribe. > To unsubscribe from this group and all its topics, 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. 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