On Dec 21, 2008, at 6:44 PM, Jason wrote:
Why doesn't (count (doall (range 10000000000))) cause an out-of memory error? doall says it causes the entire seq to reside in memory at one time, yet:
(range n) produces an object that is a seq, not just one that's seq- able. Its "rest" operation is not implemented using lazy-cons, instead it returns an object that implements the rest seq in a self-contained way: a Range object that starts one increment higher. (see clojure.lang.Range, implementation of first and rest)
Holding onto the head in this case, does not keep a realized chain of objects in memory. Instead it holds the first one only. Subsequent "rests" are generated one by one during the doall and then discarded.
Your map example turns this into a chain of lazy-cons objects with the associated much greater memory use.
The doc for doall should probably be updated to say something along the lines of "it may cause the sequence to reside in memory all at once".
--Steve
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