Very interesting indeed. I am not sure I understand completely, but by intuition I presume that the recursive call actually creates a new heap allocated LazySeq (with the function definition inside) . So is there some help from the compiler for this? How does the recursive call suddenly transfers into a call to a LazySeq object ?
On Nov 25, 6:29 pm, David Brown <cloj...@davidb.org> wrote: > On Wed, Nov 25, 2009 at 12:10:36AM -0800, Gabi wrote: > >How come that infinite recursions under lazy-seq won't crash the > >program, whilst regular infinite recursion would crash the program ? > >What's the trick ? > > >For example why doesn't the following "repeatedly" never crash? > > >(defn repeatedly > > [f] (lazy-seq (cons (f) (repeatedly f)))) > > >(last (repeatedly rand)) ;won't crash > > The JVM doesn't support tail call elimination, so an infinite > retursion causes a new stack frame for each call. The stack is fairly > small and will quickly be exhausted. > > The lazy-seq, instead, wraps the contents up into a function closure > and wraps it in a clojure.lang.LazySeq. This causes a few heap > allocations. If you were to keep the head of this chain, you would > also exhaust the heap (which would take a while longer). But since > your example is only walking along next, each item generated contains > no more references, and can be garbage collected. > > Lazy-seq basically turns a stack allocation into a heap allocation. > This both allows for not allow of the sequence to be allocated, but > allows the earlier parts of the sequence to be collected. > > Try: > > (let [r (repeatedly rand)] > (last r) > r) > > which will try to keep the whole sequence in memory, and eventually > exhaust your heap. > > David -- 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
