From bourassa@magma.ca Mon Nov  4 17:13:12 2002
From: Gregory Bourassa <bourassa@magma.ca>
To: ciao-users@cliplab.org
Subject: Re: curly braces
Date: Mon, 04 Nov 2002 17:12:39 +0100
Message-ID: <15814.40055.778902.180734@salmon.ls.fi.upm.es>
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John,

They are used in Definite Clause Grammar (DCG) clauses to enclose a
Prolog expression (term) which will not be translated by the grammar
translation.  As you may know, the DCG clauses are translated into
plain Prolog clauses which take care of handling the lists of input
tokens over which the grammar will be applied.  This saves you writing
the list-handling part of things, which can be boring and error-prone.
The terms enclosed in curly brackets are included in these translated
clauses, but without change.  You generally use them to state logical
relations about things in the grammar.

Regards.

Gregory Bourassa

On Nov 04, "John O'Gorman" <john.ogorman(a)zombie.co.nz> wrote:
>=20
>=20
> Hi people
>=20
> Can anyone tell me the meaning of {} in prolog.
> Claudi Marcus has examples of terms within curly
> braces in her book. Also Prolog: The Standard=20
> has then in its syntax diagrams.
>=20
> But I cannot find any explanation of their meaning.
>=20
> John O'Gorman
> =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
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>=20
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
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=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
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From jorgemp@icmc.usp.br Tue Nov  5 11:42:19 2002
From: "Jorge M. Pelizzoni" <jorgemp@icmc.usp.br>
To: ciao-users@cliplab.org
Subject: RE: curly braces
Date: Tue, 05 Nov 2002 11:41:25 +0100
Message-ID: <15815.41045.591652.710920@salmon.ls.fi.upm.es>
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What John said is quite true, but not quite complete. Each pair of curly
braces actually act as a kind of exceptional predefined unary operator
and, just like every other operator in Prolog (whether predefined or
not), MEANS but a (very convenient) notational aid for writing terms.
So, the meaning of=20

	{a, b, c}

is simply a term of functor '{}' and arity 1 whose ONLY argument is the
term

	','(a, ','(b, c))

You can attest that by querying=20

	?- {a, b, c} =3D '{}'(X).
	?- functor({a, b, c}, F, A).

Notice that the AND comma (not the ARGUMENT DELIMITER comma) is itself a
binary operator that just "happens" to have an associated ','/2
predicate implementing logical conjunction. So, finally cueing in John,
the thing is that the DCG "parser" simply "happens" to render a special
meaning to '{}'/1 terms. But you are still allowed to render them any
other meaning you devise, provided this happens out of the DCG scene.

Cheers,

Jorge.

> -----Original Message-----
> From: owner-ciao-users(a)clip.dia.fi.upm.es [mailto:owner-ciao-
> users(a)clip.dia.fi.upm.es] On Behalf Of Gregory Bourassa
> Sent: Monday, November 04, 2002 1:13 PM
> To: John O'Gorman; Prolog
> Subject: Re: curly braces
>=20
>=20
> John,
>=20
> They are used in Definite Clause Grammar (DCG) clauses to enclose a
> Prolog expression (term) which will not be translated by the grammar
> translation.  As you may know, the DCG clauses are translated into
> plain Prolog clauses which take care of handling the lists of input
> tokens over which the grammar will be applied.  This saves you writing
> the list-handling part of things, which can be boring and error-prone.
> The terms enclosed in curly brackets are included in these translated
> clauses, but without change.  You generally use them to state logical
> relations about things in the grammar.
>=20
> Regards.
>=20
> Gregory Bourassa
>=20
> On Nov 04, "John O'Gorman" <john.ogorman(a)zombie.co.nz> wrote:
> >
> >
> > Hi people
> >
> > Can anyone tell me the meaning of {} in prolog.
> > Claudi Marcus has examples of terms within curly
> > braces in her book. Also Prolog: The Standard
> > has then in its syntax diagrams.
> >
> > But I cannot find any explanation of their meaning.
> >
> > John O'Gorman
> >
>

=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D
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