arithmetic.pl -- Extensible arithmetic
This module provides a portable partial replacement of SWI-Prolog's user-defined arithmetic (evaluable) functions. It defines the compatibility directive arithmetic_function/1 and support for both runtime and compile-time evaluation of expressions that are a mixture between Prolog predicates used as functions and built-in evaluable terms.
- arithmetic_function(:NameArity) is det
- Declare a predicate as an arithmetic function. The function is
visible in the module in which it is defined as well as modules that
import the implementation predicate or inherit from this module. For
example:
:- use_module(library(arithmetic)). :- arithmetic_function(mid/2). mid(A,B,C) :- C is (A+B)/2.
After which we may call
?- A is mid(3,5).
, resulting inA = 4
.The implementation uses goal_expansion/2 to rewrite an arithmetic expression using user functions into a conjunction of arithmetic evaluation and predicate calls. This implies that the expression must be known at compile time. Runtime evaluation is supported using arithmetic_expression_value/2.
- arithmetic_expression_value(:Expression, -Result) is det
- True when Result unifies with the arithmetic result of evaluating Expression.