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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.
:- 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 in A = 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.