[m-users.] Building a serialiser with an additional state

[Volker Wysk]

Am Samstag, den 08.05.2021, 16:33 +0100 schrieb Sean Charles
(emacstheviking):
> What line is line 33 Volker?

It's the line "num(Nr, !St)," at the beginning of the second clause of
"number".

> It smells like accumulator passing, I had a similar issue a few days ago
> porting old school(!) Prolog DCG rules… in Mercury you have to have an in
> and an out, if you haven’t read calculator.m, do so. My solution to my
> problem is in the mail list somewhere!

This is from calculator.m:

:- pred fullexpr(expr::out, list(char)::in, list(char)::out) is semidet.

This is just a regular parser. I have no problems with that (wrote a really
big one lately). What I'm trying to build is a serialiser, which needs to
have an additional state.

Regards,
Volker

> > On 8 May 2021, at 16:30, Volker Wysk <post at volker-wysk.de> wrote:
> > 
> > Hi!
> > 
> > I'm trying to build an example of a serialiser, using DCGs, which has an
> > additional state, which needs to be threaded through. This is the code:
> > 
> > 
> > -----
> > % Number the elements of a list
> > :- pred number(list(string)::in, 
> >               int::in, int::out, 
> >               list(string)::out, list(string)::in)
> >   is det.
> > 
> > 
> > number([], !_St) -->
> >    [].
> > 
> > number([T|Ts], !St) -->
> >    num(Nr, !St),
> >    [ int_to_string(Nr), T ],
> >    count(!St),
> >    number(Ts, !St).
> > 
> > 
> > % Get the number inside the state
> > :- pred num(int::out, 
> >               int::in, int::out, 
> >               list(string)::out, list(string)::in)
> >   is det.
> > 
> > num(StIn, StIn, StIn) -->
> >    [].
> > 
> > 
> > % Increase the number inside the state by 1
> > :- pred count(int::in, int::out, 
> >              list(string)::out, list(string)::in)
> >   is det.
> > 
> > count(StIn, StIn+1) -->
> >    [].
> > -----
> > 
> > The result should be like this:
> > 
> > number(["foo","bar","baz"], 
> >       0, 3, 
> >       ["0","foo","1","bar","2","baz"], [])
> > 
> > In Prolog, exactly the same code does work (after expanding the state
> > variable syntax).
> > 
> > But in Mercury, this won't compile. I get this compiler message:
> > 
> > -----
> > number.m:033: In clause for `number(in, in, out, out, in)':
> > number.m:033:   in argument 5 of call to predicate `number.num'/5:
> > number.m:033:   mode error: variable `DCG_1' has instantiatedness `free',
> > number.m:033:   expected instantiatedness was `ground'.
> > -----
> > 
> > I vaguely understand the error message. When using "-E", the message gets
> > really long. But I don't know how it should be done...
> > 
> > 
> > Cheers,
> > Volker
> > _______________________________________________
> > users mailing list
> > users at lists.mercurylang.org
> > https://lists.mercurylang.org/listinfo/users
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