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From: dahmen@ecrc.de (Michael Dahmen)
Newsgroups: comp.lang.prolog
Subject: Re: Trapping interrupts cleanly
Message-ID: <1991Jun3.071610.23645@ecrc.de>
Date: 3 Jun 91 07:16:10 GMT
References: <9105310922.AA22183@ucbvax.Berkeley.EDU>
Sender: news@ecrc.de
Reply-To: dahmen@ecrc.de (Michael Dahmen)
Organization: European Computer-Industry Research Centre, Munich
Lines: 46

> Article 3462 in comp.lang.prolog:
> From: popx@vax.ox.ac.uk (Jocelyn Paine)

> I'd be interested to know whether there are any Prologs that handle
> interrupts in a way that doesn't destroy functional purity.

> Now, I'm using Poplog Prolog. In this, you can nominate a predicate to
> be obeyed whenever CONTROL-C is typed. (For those who know Pop-11, you
> do so by assigning to the system procedure "interrupt"). But the only
> way to pass the State to the interrupt predicate is by doing something
> like
>     mainloop( State ) :-
>         read_command( C ),
>         retractall( interrupt_goal(_) ),
>         assert( interrupt_goal( mainloop(State) ),
>         obey( C, State, NewState ),
>         mainloop( NewState ).
> and arranging for the interrupt predicate to do
>         interrupt_goal( G ),
>         call( G ).

> The only other Prolog I've used extensively is ESL Prolog-2, and it
> doesn't provide any cleaner solution to this problem. Are there any
> Prologs that do? What is considered good style on those that don't?

I suggest to use a non-local exit construct (block/3, exit_block/1).

mainloop( State ) :-
        read_command( C ),
        block(obey( C, State, NewState ), abort, State = NewState),
        mainloop( NewState ).

and arranging for the interrupt predicate to do
       exit_block(abort).

I am not sure, but I think block/3 and exit_block/1 are part of the 
proposed Prolog standard, and many Prolog implementation already
include them.

[Semantics: block(Goal, Tag, Recovery) calls Goal. If during evaluation
of Goal an exit_block(ExitTag) occurs, such that ExitTag and Tag unify,
then Recovery is called. Afterwards execution continues as if block/3
has been replaced by call(Recovery). If no exit_block/1 occurs, block/3
behaves like once(Goal).]

-- Michael Dahmen, ECRC, Munich