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

[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

> 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
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