Tail recursion modulo cons

Robert Virding <>
Fri Nov 22 00:51:05 CET 2002


Apart form Richards comments below another is that this could play havoc with error handling where you throw incomplete terms. I think.

Robert

----- Original Message ----- 
From: "Richard Carlsson" <>
To: "James Hague" <>
Cc: <>
Sent: Thursday, November 21, 2002 9:52 AM
Subject: Re: Tail recursion modulo cons


> 
> On Wed, 20 Nov 2002, James Hague wrote:
> 
> > This is a compiler optimization that effectively makes functions like the
> > following tail recursive:
> > 
> > map(F, [H|T]) ->
> >     [F(H)|map(F, T)];
> > map(_, []) -> [].
> > 
> > [...]
> > Has anyone ever looked into this for Erlang?
> 
> The actual rewriting is not that difficult; in Erlang, it would be
> something like this (assuming we had a 'set_tail' operation):
> 
> map(F, [H|T]) ->
>     Cons = [F(H) | []],
>     map(F, T, Cons);
> map(_, []) -> [].
> 
> map(F, [H|T], Cons) ->
>     NewCons = [F(H) | []],
>     set_tail(Cons, NewCons),
>     map(F, T, NewCons);
> map(_, [], Cons) ->
>     set_tail(Cons, []).
> 
> (Note that the tails of allocated cons cells must be initialised,
> e.g. to '[]', so that if garbage collection happens during execution,
> it will not find random bits in the tail. Thus, we get a penalty of an
> extra write for each cons cell.)
> 
> The big obstacle for making this work is that the abstract machine and
> garbage collector must be extended to handle write barriers. Right now,
> it is necessary that there are never any pointers from an older
> generation to a younger one. Now suppose a GC happens during execution
> of the above 'map'. Then, a pre-allocated cons cell could be moved to
> an older generation. When execution continues, it will allocate a new
> cons cell on the youngest generation, and insert a pointer from the
> old cell to the new one. The current garbage collector will not realise
> that the new cell is live, when the next collection happens. Disaster
> follows.
> 
> The quick summary is that it's not just a code transformation, but
> also requires support in the runtime environment. (It might happen
> some day, though, since there are several reasons for wanting to
> have write barriers.)
> 
> /Richard
> 
> 
> Richard Carlsson ()   (This space intentionally left blank.)
> E-mail:  WWW: http://user.it.uu.se/~richardc/
> 
> 



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