Re: Nested conditional compilation

On 20/01/2014, at 12:16 AM, Steve Moyle wrote:

> Hi, is it possible to perform nested conditional compilation?

Did you _try_ it?

When I did in 6.5, seriously weird things happened.
Perhaps it works in later versions.

It can always be simulated:

:- if(G1).
   B1.
   :- if(G2).
      B12.
   :- else.
      B1_2.
   :- end.
:- else.
   B_1.
   :- if(G2).
      B_12.
   :- else.
      B_1_2.
   :- end.
:- end.

===>

:- if(G1).
   B1.
:- end.
:- if((G1, G2)).
   B12.
:- else.
   B1_2.
:- end.
:- if(\G1).
   B_1,
:- end.
:- if((\G1,G2)).
   B_12.
:- else.
   B_1_2.
:- end.

For the record, here's what 6.5.1 did.

% cat ab.pl
a :- true.

b :- fail.

<EOF>
% cat abc.pl
:- if(a).
   p(a). 
   :- if(b).
      p(a+b).
   :- else.
      p(a-b).
   :- end.   
:- else.  
   p(-a).
   :- if(b).
      p(-a+b).
   :- else.
      p(-a-b).
   :- end.   
:- end.   
<EOF>
% swipl
Welcome to SWI-Prolog (Multi-threaded, 64 bits, Version 6.5.0)

?- [ab].
% ab compiled 0.00 sec, 3 clauses
true.

?- [abc].
ERROR: /home/cshome/o/ok/abc.pl:7:
        catch/3: Undefined procedure: end/0
Warning: /home/cshome/o/ok/abc.pl:7:
        Goal (directive) failed: user:end
ERROR: /home/cshome/o/ok/abc.pl:17:
        Unterminated conditional compilation from abc.pl:12
% abc compiled 0.01 sec, 1,316 clauses
true.

?- listing(p/1).
ERROR: [Thread 1]: exception handler failed to define listing/1

ERROR: [Thread 1]: exception handler failed to define listing/1

   Call: (6) listing(p/1) ? abort
% Execution Aborted
ERROR: load_files/2: No permission to load source `/opt/local/lib/swipl-6.5.0/library/prolog_history.pl' (Non-module file already loaded into module $toplevel; trying to load into prolog_history)
ERROR: setup_history/0: Undefined procedure: prolog_history:prolog_history/1