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 lead to hard to
understand the limits of your syntax. To ease the pain, as of SWI-Prolog
3.3.0, 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.
This is an extension specific to SWI-Prolog and the advised mechanism of
portability is not an important concern.
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 to other Prolog systems:
:- module(prove,
[ prove/1
]).
:- op(900, xfx, user:(=>)).
Unlike what many users think, operators and quoted atoms have no relation: defining an atom as an operator does not influence parsing characters into atoms and quoting an atom does not stop it from acting as an operator. To stop an atom acting as an operator, enclose it in braces like this: (myop).
xf, yf,
xfx, xfy, yfx, yfy, 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 brackets ( ... ) has
precedence 0.
The predefined operators are shown in table 4. Note that all operators can be redefined by the user.
| 1200 | xfx | -->, :- |
| 1200 | fx | :-, ?- |
| 1150 | fx | dynamic, discontiguous, initialization, module_transparent, multifile, thread_local, volatile |
| 1100 | xfy | ;, | |
| 1050 | xfy | ->,
op*-> |
| 1000 | xfy | , |
| 900 | fy | \+ |
| 900 | fx | ~ |
| 700 | xfx | <, =, =.., =@=, =:=, =<, ==,
=\=, >, >=, @<, @=<, @>,
@>=, \=, \==, is |
| 600 | xfy | : |
| 500 | yfx | +, -, /\, \/, xor |
| 500 | fx | +, -, ? |
| 400 | yfx | *, /, //, rdiv,
<<, >>, mod, rem |
| 200 | xfx | ** |
| 200 | xfy | ^ |
| 200 | fy | \ |
| Table 4 : System operators |