5.3 Syntax changes

5.3.1 Operators and quoted atoms

As of SWI-Prolog version 7, quoted atoms loose their operator property. This means that expressions such as A = 'dynamic'/1 are valid syntax, regardless of the operator definitions. From questions on the mailinglist this is what people expect.^{127We believe that most users expect an operator declaration to define a new token, which would explain why the operator name is often quoted in the declaration, but not while the operator is used. We are afraid that allowing for this easily creates ambiguous syntax. Also, many development environments are based on tokenization. Having dynamic tokenization due to operator declarations would make it hard to support Prolog in such editors.} To accomodate for real quoted operators, a quoted atom that needs quotes can still act as an operator.^{128Suggested by Joachim Schimpf.} A good use-case for this is a unit library^{129https://groups.google.com/d/msg/comp.lang.prolog/ozqdzI-gi_g/2G16GYLIS0IJ}, which allows for expressions such as below.

?- Y isu 600kcal - 1h*200'W'.
Y = 1790400.0'J'.

5.3.2 Compound terms with zero arguments

As of SWI-Prolog version 7, the system supports compound terms that have no arguments. This implies that e.g., name() is valid syntax. This extension aims at functions on dicts (see section 5.4) as well as the implementation of domain specific languages (DSLs). To minimise the consequences, the classic predicates functor/3 and =../2 have not been modified. The predicates compound_name_arity/3 and compound_name_arguments/3 have been added. These predicates operate only on compound terms and behave consistently for compounds with zero arguments. Code that generalises a term using the sequence below should generally be changed to use compound_name_arity/3.

    ...,
    functor(Specific, Name, Arity),
    functor(General, Name, Arity),
    ...,

Replacement of =../2 by compound_name_arguments/3 is typically needed to deal with code that follow the skeleton below.

    ...,
    Term0 =.. [Name|Args0],
    maplist(convert, Args0, Args),
    Term =.. [Name|Args],
    ...,

For predicates, goals and arithmetic functions (evaluable terms), <name> and <name>() are equivalent. Below are some examples that illustrate this behaviour.

go() :- format('Hello world~n').

?- go().
Hello world

?- go.
Hello world

?- Pi is pi().
Pi = 3.141592653589793.

?- Pi is pi.
Pi = 3.141592653589793.

Note that the cannonical representation of predicate heads and functions without arguments is an atom. Thus, clause(go(), Body) returns the clauses for go/0, but clause(-Head, -Body, +Ref) unifies Head with an atom if the clause specified by Ref is part of a predicate with zero arguments.

5.3.3 Block operators

Introducing curly bracket and array subscripting.^{130Introducing block operators was proposed by Jose Morales. It was discussed in the Prolog standardization mailing list, but there were too many conflicts with existing extensions (ECLiPSe and B-Prolog) and doubt about their need to reach an agreement. Increasing need to get to some solution resulted in what is documented in this section. These extensions are also implemented in recent versions of YAP.} The symbols [] and {} may be declared as an operator, which has the following effect:

[ ]

This operator is typically declared as a low-priority yf postfix operator, which allows for array[index] notation. This syntax produces a term []([index],array).

{ }

This operator is typically declared as a low-priority xf postfix operator, which allows for head(arg) { body } notation. This syntax produces a term {}({body},head(arg)).

Below is an example that illustrates the representation of a typical `curly bracket language' in Prolog.

?- op(100, xf, {}).
?- op(100, yf, []).
?- op(1100, yf, ;).

?- displayq(func(arg)
            { a[10] = 5;
              update();
            }).
{}({;(=([]([10],a),5),;(update()))},func(arg))