Dear Jose,

thank you for your answer. I'll have a look at the paper you suggested me.

Kind regards, Silvia Umiliacchi

----Messaggio originale---- Da: jfran(a)clip.dia.fi.upm.es Data: 28/06/2011 18.24 A: "silviaumiliacchi(a)libero.it"<silviaumiliacchi(a)libero.it> Cc: <ciao-users(a)clip.dia.fi.upm.es> Ogg: Re: [Ciao-users] Question about tail call optimization

Dear Silvia,

Last call optimization (LCO) removes frame environments when possible, thus transforming some recursions into loops. That optimization is done at compile time when possible. There are many papers in the literature where you can find a more detailed description (H. Ait-Kaci. Warren's Abstract Machine, A Tutorial Reconstruction). Basically, given a clause:

p :- g1, g2, ..., gn1, gn.

the compiler translates it into code (e.g., bytecode) that, between 'gn1' and 'gn', makes sure that all the arguments required for 'gn' live in temporal registers, removes the environment frame, and jumps to 'gn'.

In order to reason about catch/3 and throw/1 you need some detailed description of their semantic. Suppose a clause like:

foo :- ..., catch(bar, ..., ...).

If your question is whether LCO is able to remove the environment created for 'foo' before calling 'bar', the answer is no. At least, you need to 'uninstall' the exception handler when returning from 'bar'. To do that, you need to keep some information in the enviroment. I do not think that it could be optimized (at least I am not aware of any Prolog system which does).

Cheers

-- Jose