
eval(+Expression, -Result)

   Used to evaluate eval/1 terms in arithmetic expressions.

Arguments
   Expression          An arithmetic expression.
   Result              Output: a number.

Type
   Arithmetic

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 --- Expression is uninstantiated.
    21 --- An evaluation predicate in the expression is not defined.
    24 --- 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
