[m-users.] Problem with XML extra library

[Volker Wysk]

Hi

I'm trying to acquaint myself with the XML library in "extras".

There's "tryit.m", which is a simple example of using the library, which
compiles fine. It is like this:

-----
...
:- pred main(io::di, io::uo) is det.

:- implementation.
...

main -->
    io.command_line_arguments(Args),
    main(Args).

:- pred main(list(string), io, io).
:- mode main(in, di, uo) is det.

main([]) --> [].
main([File|Files]) -->
    ...
    	    { TextResult = ok(Text) },
	    pstate(mkEntity(Text), mkEncoding(utf8), init),
            ...
-----

The two "main" predicates are written in DCG syntax. The call to pstate/5 is
therefore (??) an abbreviation of

   pstate(mkEntity(Text), mkEncoding(utf8), init, !IO)

But the declaration of pstate/5 in parsing.m is:

   :- pred pstate(entity, encoding, globals, io, pstate(unit)).
   :- mode pstate(in, in, in, di, puo) is det.

So the type of the output state variable is "pstate(unit)". But it is "io",
when main/3 is called. The types don't match. 

I've tried it with this little program:

...
main(!IO) :-
    pstate(mkEntity("Das ist mein Text."), mkEncoding(utf8), init, !IO).

The result is like expected: 

v.m:017: In clause for predicate `main'/2:
v.m:017:   in argument 5 of call to predicate `parsing.pstate'/5:
v.m:017:   type error: variable `STATE_VARIABLE_IO' has type
v.m:017:     `io.state',
v.m:017:   expected type was
v.m:017:     `parsing.pstate(unit.unit)'.

So why on earth does it work in tryit.m ?


Cheers,
Volker
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