okay, so I am using prolog to build a simple xml parser. And I have the following xml file:
<ip> <line> 7 </line> <envt> p1:1 in main:1 </envt> </ip>
<contour>
<name> main:1 </name>
<items>
<item> <var> x:int </var> <val> 2 </val> </item>
<item> <var> y:int </var> <val> 2 </val> </item>
</items>
<rpdl> system </rpdl>
<nested>
<contour>
<name> p1:1 </name>
<items>
<item> <var> y:int </var> <val> 0 </val> </item>
<item> <var> q:proc </var> <val> p2 in main:1 </val> </item>
</items>
<rpdl> <line> 21 </line> <envt> main:1 in root:1 </envt> </rpdl>
</contour>
</nested>
</contour>
</program_state>
and in Prolog I use the following DCG rules:
xml([E]) --> element(E).
xml([E|L]) --> element(E), xml(L).
element(E) --> begintag(N), elements(L), endtag(N), {E =.. [N|L]}.
elements(L) --> xml(L).
elements([E]) --> [E].
begintag(N) --> ['<', N, '>'].
endtag(N) --> ['<', '/', N, '>'].
so the rules can't handle things like "p1:1 in main:1", "x:int", "main:1". I actually tried to change those things into "p1", "x", "main", and the parser works perfectly fine. Now what rules should I add so that the parser could handle the irregular tokens?
The parse tree will be something like this:
program_state(
ip(line(7), envt(p1:1 in main:1)),
contour(name(main:1),
items(item(var(x:int),val(2)),
item(var(y:int),val(2))),
rpdl(system),
nested(contour( name(p1:1),
items(item(var(y:int),val(0)),
item(var(q:proc),val(p2 in main:1))),
rpdl(line(21),envt(main:1 in root:1)),
))))
and the following is what I get:
program_state(
ip(line(7), envt(p1)),
contour(name(main),
items(item(var(x), val(2)),
item(var(y), val(2))),
rpdl(system),
nested(contour(name(p1),
items(item(var(y), val(0)),
item(var(q), val(p2))),
rpdl(line(21), envt(main))
)))).
I tokenized the XML, fed it to the parser and it worked fine. Verify the input: Symbols with a colon(:) in them need to be quoted; otherwise they represent module_name:module_specific_symbol. Here's the input:
?- listing(input).
input([<, program_state, >, <, ip, >, <, line, >, '7', <, /, line, >, <, envt, >, ['p1:1', in, 'main:1'], <, /, envt, >, <, /, ip, >, <, contour, >, <, name, >, 'main:1', <, /, name, >, <, items, >, <, item, >, <, var, >, 'x:int', <, /, var, >, <, val, >, '2', <, /, val, >, <, /, item, >, <, item, >, <, var, >, 'y:int', <, /, var, >, <, val, >, '2', <, /, val, >, <, /, item, >, <, /, items, >, <, rpdl, >, system, <, /, rpdl, >, <, nested, >, <, contour, >, <, name, >, 'p1:1', <, /, name, >, <, items, >, <, item, >, <, var, >, 'y:int', <, /, var, >, <, val, >, '0', <, /, val, >, <, /, item, >, <, item, >, <, var, >, 'q:proc', <, /, var, >, <, val, >, [p2, in, 'main:1'], <, /, val, >, <, /, item, >, <, /, items, >, <, rpdl, >, <, line, >, '21', <, /, line, >, <, envt, >, ['main:1', in, 'root:1'], <, /, envt, >, <, /, rpdl, >, <, /, contour, >, <, /, nested, >, <, /, contour, >, <, /, program_state, >]).
true.
A listing of the how the parser is invoked:
?- listing(run).
run :-
consult('input.db'),
input(A),
phrase(xml(B), A),
write(B),
nl.
true.
A listing of the parser run:
?- run.
% input.db compiled 0.00 sec, 2,768 bytes
[program_state(ip(line(7),envt([p1:1,in,main:1])),contour(name(main:1),items(item(var(x:int),val(2)),item(var(y:int),val(2))),rpdl(system),nested(contour(name(p1:1),items(item(var(y:int),val(0)),item(var(q:proc),val([p2,in,main:1]))),rpdl(line(21),envt([main:1,in,root:1]))))))]
true