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Used to evaluate eval/1 terms in arithmetic expressions.
- An arithmetic expression.
- Output: a number.
This is one of the predicates used by the ECLiPSe compiler to expand
arithmetic expressions. If an expression contains a subexpression that
is not known at compile time, it must be wrapped in eval/1, e.g.
X is eval(Expr)+1
This will be compiled into the sequence
and eval/2 will interpret the expression Expr at runtime.
Modes and Determinism
This predicate is sensitive to its module context (tool predicate, see @/2).
- (4) instantiation fault
- Expression is uninstantiated.
- (21) undefined arithmetic expression
- An evaluation predicate in the expression is not defined.
- (24) number expected
- Expression is not a valid arithmetic expression.
% Given the program code:
p(Number, Result) :- Result is Number + 1.
q(Expression, Result) :- Result is eval(Expression) + 1.
% Only q/1 accepts expressions:
?- p(2+3, R).
number expected in +(2 + 3, 1, _177)
?- q(2+3, R).
R = 6
Yes (0.00s cpu)
is / 2