On Aug 13, 3:38 am, Stuart Halloway <stuart.hallo...@gmail.com> wrote: > > Stu, (or anybody) I'd like to ask about a variation on this > > point. How do you handle the case where you have a general > > function that works for every type you'd like to implement a > > protocol for (thus not technically needing to be in a protocol), > > but maybe 1 or 2 of the many types have more efficient > > implementations possible? Do you just suck it up and copy and > > paste the general function around? Or is there a better way? Maybe > > the new case function? > > This might be a job for a more granular protocol. Take a look at how > granular Clojure's implementation abstractions are (even prior to > the introduction of protocols). The Java interfaces often have 0, 1, > or 2 methods.
The scenario I'm thinking of is when you've already published a protocol, and now want to extend it. Instead, you could just create a new (granular) protocol, but that does seem to lead inexorably to the IFoo, IFoo2, IFoo3, situation. A more concrete example: say I've defined a protocol for AST nodes in 1.0 of a library, and later when developing 2.0 I discover it would have been a good idea to have a "pretty-print" method on nodes to show human-readable output. If the protocol had Trait-like characteristics I could add pretty-print to the protocol, with a default implementation that just prints the fields of the node, but override that with a better implementation for some of the new record types I'm including in 2.0. But I can't do that because, unless I've had the foresight to make sure clients always mix in a base set of (partial) defaults that I provide (which may be initially empty), then things break down. Sean Devlin's ideas (referenced earlier on the thread) on making it easy to mix in defaults would be great for this, but they don't really address the issue I'm looking at here. Following Stuart's suggestion, I *could* just add a protocol called "PrettyPrintable" with one method and implement it on some of the new node types, but now I can't just call "pretty-print" on any node: I need to write another function that checks if it's a PrettyPrintable first and calls something default if not. I've had to do this sort of thing many, many times in Java: one of the reasons I got excited about multimethods in Clojure is that they allow me to transparently extend the system after the fact with no such ugliness. Thanks to the superpower that is macro, I'm sure I could make a defprotocol+ and a extend+ that do this, just wanted to check that there's really no better way. Cheers, Matthew. -- 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
