Re: Iteratively calling Prolog from C

[Daniel Dudley]

Daniele Peri wrote:
Lindsey Spratt wrote:
>
> On Wednesday, July 2, 2003, at 03:30  AM, Daniele Peri wrote:
>
>> I made some tests about iteratively calling prolog from a foreign
>> C function. The idea is to improve either performance or memory
>> footprint of very deep recursion calls. This is also needed as
>> gprolog doesn't seem to optimize tail recursion. I didn't delve
>> into gprolog source but I have not find any reference to such
>> optimization in the documentation.
>
> gprolog *does* "optimize tail recursion". It uses the Last Call
> Optimization (LCO). I looked at the generated WAM code, and gprolog
> appears to follow the LCO as described by Hassan Aït-Kaci in
> "Warren's Abstract Machine: A Tutorial Reconstruction" (MIT Press,
> 1991). Specifically, the "call;deallocate" at the end of the body
> is replaced by "deallocate;execute". This can work well for a tail
> recursive predicate, leading to constant stack size. Unfortunately,
> there is nothing to optimize the use of heap storage, so a simple
> recursive predicate may exhaust the heap. For instance, consider
> the following predicate to calculate the sum of the series of
> integers from 1 to N

Subtle, doubtful, distinction that, unfortunately, does not solve
the problem.
My little hack would do what I need until the main problem is solved
if it only worked.
I would really appreciate any useful comments.

Well, without knowing exactly what you are trying to
accomplish with recursion, may I venture to suggest that
you replace the recursive loop with a repeat-fail type loop
(using accumulator variables to store results over
backtracking)? Here you get garbage collection for free.
You'll find a number of examples on my website:

    http://home.chello.no/~dudley/

Daniel