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edw
Posted: Thu Oct 12, 2006 11:26 am Reply with quote
Joined: 08 Oct 2006 Posts: 2
Hi,
I have a Smalltalk background and got interested
in Erlang recently.
Having tried some tutorials and thought about
the language concepts, it occured to me that it is
impossible in Erlang to have cyclic data structures
(e.g. a list containing itself).
Is this really true, or am I missing something?
This would have a tremendous impact on the garbage
collector, I guess.
Is there a documentation somewhere about the
details of the Erlang garbage collector?
Thanks,
Wolfgang
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francesco
Posted: Tue Oct 17, 2006 9:18 am Reply with quote
User Joined: 07 Jul 2006 Posts: 249 Location: London
Hi WOlfgan,

The closest you can get to cyclic datastructures is using funs in the tail of the list.

Code:
-module(f).
-compile(export_all).

from(H) ->
    [H|fun() -> from(H+1) end].

2> [H|T] = f:from(1).
[1|#Fun<f.0.61259012>]
3> [H2|T2] = T().
[2|#Fun<f.0.61259012>]


Regarding the GC, it is a generationincremental GC which is run on a per process basis when needed. There were many implementations of the VM in the early days (VEE, JAM, BEAM, plus Mike WIlliam's own, which was before my time), all experimenting with different variants of memory management. On top of that, tricks such as deallocating all the memory when a process dies, shared binary heap, no shared memory gives the concurrency and speed we see today, even if running in an interpreted environment. The best one won, andd has since constantly been improved. I am unsure of where it is documented, if at all. Your best bet is to scrounge the erlang mailing list archives Smile

Francesco
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edw
Posted: Thu Oct 19, 2006 7:44 am Reply with quote
Joined: 08 Oct 2006 Posts: 2
Thanks Francesco,
I have thought about your example. I see that
it "feels" cyclic, but I don't recognize the cycle in
the memory structure.
With cycles in memory I mean 2 objects referencing
each other.
Thanks
Wolfgang
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vladdu
Posted: Fri Oct 20, 2006 7:07 am Reply with quote
User Joined: 28 Feb 2005 Posts: 397 Location: Gothenburg, Sweden
Hi,

You can't have real cyclic data structures in Erlang. One way to see why not, is to consider that to make a structure cyclic you need to be able to update it in place. The Erlang semantics are non-destructive, so it doesn't let you do that.

See also http://www.erlang.org/faq/x479.html, 5.6

best regards,
Vlad
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