Allows functors (names of predicates) to be defined as "operators". The use of operators allows syntax to be easier to write and read. Z is the atom that we want to be an operator, X is the precedence class (an integer), and Y the associativity specifier. e.g. op(1200,xfx,':-')
Examples
'~'(X,Y) :- X>Y-4, X<Y+4.
?- op(1000,xfx,'~').
yes
?- 4 ~ 7.
yes
?- 7 ~ 7.
yes
?- 10 ~ 7.
yes
?- 11 ~ 7.
no
?- 3 ~ 7.
noExample of invalid arguments
?- op(X,xfx,'><').
Expected Numeric but got: VARIABLE with value: X
?- op(1000,Y,'><').
Expected an atom but got: VARIABLE with value: Y
?- op(1000,xfx,Z).
Expected an atom but got: VARIABLE with value: Z
?- op(1000,zfz,'><').
Cannot add operand with associativity of: zfz as the only values allowed are: [xfx, xfy, yfx, fx, fy, xf, yf]Create some prefix and postfix operators for the later examples below.
?- op(550, fy, 'fyExample').
yes
?- op(650, fx, 'fxExample').
yes
?- op(600, yf, 'yfExample').
yes
?- op(500, xf, 'xfExample').
yesExample of nested prefix operators.
?- X = fxExample fyExample fyExample a, write_canonical(X), nl.
fxExample(fyExample(fyExample(a)))
X = fxExample fyExample fyExample a
yesExample of a postfix operator.
?- X = 123 yfExample, write_canonical(X), nl.
yfExample(123)
X = 123 yfExample
yesExample of nested postfix operators.
?- X = a xfExample yfExample yfExample, write_canonical(X), nl.
yfExample(yfExample(xfExample(a)))
X = a xfExample yfExample yfExample
yesExample of combining post and prefix operators where the postfix operator has the higher precedence.
?- X = fyExample a yfExample, write_canonical(X), nl.
yfExample(fyExample(a))
X = fyExample a yfExample
yesExample of combining post and prefix operators where the prefix operator has the higher precedence.
?- op(699, fy, 'fyExampleB').
yes
?- X = fyExampleB a yfExample, write_canonical(X), nl.
fyExampleB(yfExample(a))
X = fyExampleB a yfExample
yesExamples of how an "x" in an associativity (i.e. "fx" or "xf") means that the argument can contain operators of only a lower level of priority than the operator represented by "f".
?- X = a xfExample xfExample.
Operator priority clash. xfExample (500) conflicts with previous priority (500) Line: X = a xfExample xfExample.
?- X = fxExample fxExample a.
Operator priority clash. fxExample (650) conflicts with previous priority (650) Line: X = fxExample fxExample a.