Hi Andy, Marking Java fields as final has semantics with respect to threading, defined in the JLS in section 17.5 (http://docs.oracle.com/javase/specs/jls/se5.0/html/memory.html#17.5)
If you do this, then it makes it possible to freely pass a "truly" immutable object instance to another thread and be guaranteed that that other thread will see the value initialized for that field in the constructor. If you don't do this, then the object can still be "effectively" immutable, which essentially means that you can pass the object to another thread, so long as you do it in a safe manner (using a volatile, or some synchronization mechanism). JCIP helps clarify all of this unfortunately complex topic. The important thing (and key to Closure), is that, if you are implementing the class that you want to be immutable, then if you can mark everything as final, then you truly achieve the benefits immutability give you with concurrency (in short, you need no synchronization whatsoever). If you fail to do this, then you have "effective" immutability, which is good, but complex and comes with caveats on how you can safely pass objects between threads. JCIP is a great book. But, the approach taken by Clojure makes a lot of the complicated concerns covered by the book largely ignorable, IMHO. On Tuesday, May 6, 2014 8:35:43 PM UTC-4, Andy Fingerhut wrote: > > Alex, I may be unfamiliar with the definitions of truly immutable and > effectively immutable being used here, but if I can mutate the contents of > a Java Object array that is a final field after an object is constructed, > does it really matter that much if it is final? It is trivially easy to > mutate using Java access. Here is the example that I mentioned earlier in > this thread, copied here for convenience: > > user=> (def v [1 2 3]) > #'user/v > user=> (class v) > clojure.lang.PersistentVector > user=> v > [1 2 3] > user=> (aset (.tail v) 1 -2) > -2 > user=> v > [1 -2 3] > > Andy > > > On Tue, May 6, 2014 at 4:49 PM, Alex Miller <al...@puredanger.com<javascript:> > > wrote: > >> The Clojure persistent data structures are truly immutable - all fields >> are final and referred objects are not mutated after construction so that >> freeze occurs. One obvious exception are the transient variants ( >> http://clojure.org/transients). You can look at the code in >> https://github.com/clojure/clojure/tree/master/src/jvm/clojure/lang - >> any of the Persistent*.java. >> >> >> On Tuesday, May 6, 2014 4:11:49 PM UTC-5, Mike Fikes wrote: >>> >>> Are the persistent immutable data structures in Clojure "truly" >>> immutable (using final fields, relying on constructor freezing), or are >>> they mean to be merely effectively immutable (as defined in JICP)? >>> >> -- >> You received this message because you are subscribed to the Google >> Groups "Clojure" group. >> To post to this group, send email to clo...@googlegroups.com<javascript:> >> Note that posts from new members are moderated - please be patient with >> your first post. >> To unsubscribe from this group, send email to >> clojure+u...@googlegroups.com <javascript:> >> 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+u...@googlegroups.com <javascript:>. >> 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.
