[ Arithmetic | Reference Manual | Alphabetic Index ]
eval(+Expression, -Result)
Used to evaluate eval/1 terms in arithmetic expressions.
- Expression
- An arithmetic expression.
- Result
- Output: a number.
Description
   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
   eval(Expr,T1), +(T1,1,X)
   and eval/2 will interpret the expression Expr at runtime.
Modes and Determinism
Modules
This predicate is sensitive to its module context (tool predicate, see @/2).
Exceptions
- (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.
Examples
   % 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)
   Abort
   ?- q(2+3, R).
   R = 6
   Yes (0.00s cpu)
See Also
is / 2