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

[Volker Wysk]

Am Samstag, den 08.05.2021, 16:49 +0100 schrieb Sean Charles
(emacstheviking):
> From what I have learned so far, all I can say is that it was expecting a
> value to have been written into that when the predicate succeeds, but
> according to its magic with the inst trees it has decided that at that
> point in time, the variable is still free i.e. has no value in it. That
> may or may not make your head worse. :D

Yes, I understand that much from the error message. But why does it work in
Prolog and not here? How should it be written?

(Please bottom-post in mailing lists.)

Bye
Volker

> 
> > On 8 May 2021, at 16:45, Volker Wysk <post at volker-wysk.de> wrote:
> > 
> > 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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