ychr-0.1.0.0: examples/fib.chr

% Fibonacci as a function, called from a constraint's rule body.
%
% Three equations: two base cases and one recursive case guarded by
% N > 1. The constraint `compute(N, R)` binds R to fib(N) using the
% `is` operator, which evaluates its right-hand side as an expression.
%
% Used by docs/tutorials/04-functions-and-types.md §1.

:- module(fib, [compute/2, fun fib/1]).
:- chr_constraint compute(int, int).
:- function fib(int) -> int.

fib(0) -> 0.
fib(1) -> 1.
fib(N) | N > 1 -> fib(N - 1) + fib(N - 2).

compute(N, R) <=> R is fib(N).