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# 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

- eval(+, -) is det

### 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