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%  Copyright 1998-2021 Paulo Moura <pmoura@logtalk.org>
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:- object(states).

	:- info([
		version is 1:0:0,
		author is 'Paulo Moura',
		date is 2019-01-31,
		comment is 'Example of threading state between predicate calls using Definite Clause Grammars (DCGs).'
	]).

	:- public(convert/2).
	:- mode(convert(+atom, -atom), one).
	:- info(convert/2, [
		comment is 'Converts a floating point number into an integer number by applying a sequence of operations.',
		argnames is ['Float', 'Integer']
	]).

	convert(Float, Integer) :-
		phrase(steps, Float, Integer).

	steps -->
		square,
		half,
		round.

	% we use a lambda expression to access the implicit
	% difference-list in each grammar rule

	square -->
		call([Number, Double]>>(Double is Number * Number)).

	half -->
		call([Number, Half]>>(Half is Number / 2.0)).

	round -->
		call([Float, Integer]>>(Integer is round(Float))).

	% alternative formulation where the individual steps
	% are defined as predicates instead of non-terminals
	%
	% the call//1 built-in meta non-terminal is used to
	% avoid hard-coding assumptions about how grammar
	% rules are compiled into clauses

%	steps -->
%		call(square),
%		call(half),
%		call(round).
%
%	square(Number, Double) :-
%		Double is Number*Number.
%
%	half(Number, Half) :-
%		Half is Number / 2.0.
%
%	round(Float, Integer) :-
%		Integer is round(Float).

:- end_object.
