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From: Mild Shock <janburse@fastmail.fm>
Newsgroups: comp.lang.prolog
Subject: Re: DCG restrictions on the left-hand side [Novacore]
Date: Wed, 31 Jul 2024 08:07:22 +0200
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I think its worth adopting this behaviour:

/* Scryer Prolog */
?- a-->b.
    error(existence_error(procedure,(-->)/2),(-->)/2).
?- assertz((a-->b)).
    error(permission_error(modify,static_procedure,(-->)/2),assertz/1).
?- a-->b.
    error(existence_error(procedure,(-->)/2),(-->)/2).

Mild Shock schrieb:
> Although SWI-Prolog implements a lot of checks.
> It still allows this one:
> 
> ?- a-->b.
> ERROR: Unknown procedure: (-->)/2
> 
> ?- assertz((a-->b)).
> true.
> 
> ?- a-->b.
> true.
> 
> Scryer Prolog managed to prevent this, in that
> the assertz/1 throws an error.
> 
> Think will finally adopt this behaviour to
> prevent the end-user from accidentially
> 
> fiddling with (-->)/2.
> 
> Mild Shock schrieb:
>>
>> This was discussed here:
>>
>> Definite Clause Grammars Harmonization Proposal
>> Last edit jschimpf? October 20, 2016, at 12:42 AM
>> https://eclipseclp.org/wiki/Prolog/DCG
>>
>> Not sure why Scryer Prolog doesn't implement it.
>>
>> Mild Shock schrieb:
>>>
>>> In Novacore we are lucky, its a side effect of
>>> phrase translating the head and a multi-file check:
>>>
>>> ?- [user].
>>> p;q --> r.
>>> Fehler: Kann Prädikat ;/2 nicht umdefinieren, nicht als Mehrdateien 
>>> markiert.
>>>
>>> But might work out differently if a DCG is expanded
>>> and asserted. Ok, its actually not so bad:
>>>
>>> ?- expand_term((p;q --> r), X), assertz(X).
>>> Fehler: Kann Prädikat ;/2 nicht aktualisieren.
>>>      user auf 1
>>>
>>>
>>> Mild Shock schrieb:
>>>> Guess how many newer Prolog systems allow this here:
>>>>
>>>> ?- [user].
>>>> p;q --> r.
>>>>     true.
>>>>
>>>> On the other hand SWI-Prolog disallows it:
>>>>
>>>> ?- [user].
>>>> p;q --> r.
>>>>
>>>> ERROR: user://1:29:
>>>> ERROR:    No permission to define dcg_nonterminal `p;q'
>>>
>>
>