Operators are defined to improve the readability of source code. For
example, without operators, to write 2*3+4*5 one would have
to write +(*(2,3),*(4,5)). In Prolog, a number of operators
have been predefined. All operators, except for the comma (,) can be
redefined by the user.
Some care has to be taken
before defining new operators. Defining too many operators might make
your source‘natural’looking, but at the same time using many
operators can make it hard to understand the limits of your syntax.
In SWI-Prolog, operators are local to the module in which they are
defined. Operators can be exported from modules using a term
op(Precedence, Type, Name) in the export list as specified
by
module/2.
Many modern Prolog systems have module specific operators.
Unfortunately, there is no established interface for exporting and
importing operators. SWI-Prolog's convention has been adopted by YAP.
The module table of the module user acts as default
table for all modules and can be modified explicitly from inside a
module to achieve compatibility with other Prolog that do not have
module-local operators:
:- module(prove,
[ prove/1
]).
:- op(900, xfx, user:(=>)).
Although operators are module-specific and the predicates that define
them (op/3)
or rely on them such as current_op/3, read/1
and write/1
are module sensitive, they are not proper meta-predicates. If they were
proper meta predicates read/1
and write/1
would use the module from which they are called, breaking compatibility
with other Prolog systems. The following rules apply:
- If the module is explicitly specified by qualifying the third
argument (op/3, current_op/3)
or specifying a
module(Module) option (read_term/3, write_term/3),
this module is used.
- While compiling, the module into which the compiled code is loaded
applies.
- Otherwise, the typein module applies. This is normally
user
and may be changed using module/1.
In SWI-Prolog, a quoted atom never acts as an operator. Note
that the portable way to stop an atom acting as an operator is to
enclose it in parentheses like this: (myop). See also
section 5.3.1.
- [ISO]op(+Precedence,
+Type, :Name)
- Declare Name to be an operator of type Type with
precedence
Precedence. Name can also be a list of names, in
which case all elements of the list are declared to be identical
operators.
Precedence is an integer between 0 and 1200. Precedence 0
removes the declaration. Type is one of:
xf, yf,
xfx, xfy, yfx, fy or
fx. The‘f’indicates the position
of the functor, while
x and y indicate the position of the
arguments.‘y’should be interpreted as “on
this position a term with precedence lower or equal to the precedence of
the functor should occur” . For‘x’the
precedence of the argument must be strictly lower. The precedence of a
term is 0, unless its principal functor is an operator, in which case
the precedence is the precedence of this operator. A term enclosed in
parentheses ( ... has precedence 0.
The predefined operators are shown in table
5. Operators can be redefined, unless prohibited by one of the
limitations below. Applications must be careful with (re-)defining
operators because changing operators may cause (other) files to be
interpreted
differently. Often this will lead to a syntax error. In other
cases, text is read silently into a different term which may lead to
subtle and difficult to track errors.
- It is not allowed to redefine the comma (
',').
- The bar (
|) can only be (re-)defined as infix operator
with priority not less than 1001.
In SWI-Prolog, operators are local to a module (see also
section 6.9). Keeping
operators in modules and using controlled import/export of operators as
described with the module/2
directive keep the issues manageable. The module system
provides the operators from table
5 and these operators cannot be modified. Files that are loaded from
the SWI-Prolog directories resolve operators and predicates from this system
module rather than user, which makes the semantics of the
library and development system modules independent of operator changes
to the user module. See section
4.25 for details about the relation between operators and modules.
| 1200 | xfx | -->, :-, => |
| 1200 | fx | :-, ?- |
| 1150 | fx | dynamic, discontiguous, initialization,
meta_predicate,
module_transparent, multifile, public,
thread_local, thread_initialization, volatile |
| 1105 | xfy | | |
| 1100 | xfy | ; |
| 1050 | xfy | ->, *-> |
| 1000 | xfy | , |
| 990 | xfx | := |
| 900 | fy | \+ |
| 700 | xfx | <, =, =.., =@=, \=@=,
=:=, =<, ==,
=\=, >, >=, @<, @=<, @>,
@>=, \=, \==, as, is,
>:<, :< |
| 600 | xfy | : |
| 500 | yfx | +, -, /\, \/, xor |
| 500 | fx | ? |
| 400 | yfx | *, /, //, div, rdiv,
<<, >>, mod, rem |
| 200 | xfx | ** |
| 200 | xfy | ^ |
| 200 | fy | +, -, \ |
| 100 | yfx | . |
| 1 | fx | $ |
Table 5 : System operators
- [ISO]current_op(?Precedence,
?Type, ?:Name)
- True if Name is currently defined as an operator of type Type
with precedence Precedence. See also op/3.
Note that an
unqualified Name does not resolve to the
calling context but, when compiling, to the compiler's target module and
otherwise to the typein module. See
section 4.25 for
details.
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