%---------------------------------------------------------------------------%
% vim: ts=4 sw=4 et ft=mercury
%---------------------------------------------------------------------------%
%
% The "doubleapp" Benchmark.
% Part of the DPPD Library.
% 
% This is a naive implementation for a predicate that appends three
% lists, written using two calls to the ordinary append predicate. This
% program is inefficient because the intermediate variable Int is
% constructed by the first call to append and then traversed again by
% the second call to append.

:- module doubleapp.

:- interface.

:- pred doubleapp is semidet.

:- implementation.

:- import_module doubleapp_impl.
:- import_module list.
:- import_module run.

doubleapp :-
    double_app([], [a], [b, c], Res1),
    use(Res1),
    double_app([a, b, c], [d, e, f], [g, h, i], Res2),
    use(Res2),
    double_app([1, 5, 3, 2, 6, 3, 7, 3, 2, 1, 8, 5, 3, 5, 2, 3],
        [1, 5, 3, 2, 6, 3, 7, 3, 2, 1, 8, 5, 3, 5, 2, 3],
        [1, 5, 3, 2, 6, 3, 7, 3, 2, 1, 8, 5, 3, 5, 2, 3], Res3),
    use(Res3),
    double_app([1, 5, 3, 2, 6, 3, 7, 3, 2, 1, 8, 5, 3, 5, 2, 3],
        [1, 5, 3, 2, 6, 3, 7, 3, 2, 1, 8, 5, 3, 5, 2, 3],
        [1, 5, 3, 2, 6, 3, 7, 3, 2, 1, 8, 5, 3, 5, 2, 3], Res4),
    use(Res4).

% The partial deduction query
% 
% :- double_app(X, Y, Z, Res).
% 
% The run-time queries
% 
% :- double_app([], [a], [b, c], Res).
% :- double_app([a, b, c], [d, e, f], [g, h, i], Res).
% :- double_app([1, 5, 3, 2, 6, 3, 7, 3, 2, 1, 8, 5, 3, 5, 2, 3],
%     [1, 5, 3, 2, 6, 3, 7, 3, 2, 1, 8, 5, 3, 5, 2, 3],
%     [1, 5, 3, 2, 6, 3, 7, 3, 2, 1, 8, 5, 3, 5, 2, 3], Res).
% :- double_app([1, 5, 3, 2, 6, 3, 7, 3, 2, 1, 8, 5, 3, 5, 2, 3],
%     [1, 5, 3, 2, 6, 3, 7, 3, 2, 1, 8, 5, 3, 5, 2, 3],
%     [1, 5, 3, 2, 6, 3, 7, 3, 2, 1, 8, 5, 3, 5, 2, 3], Res).
% 
% Example solution
% 
% The following specialised program can be obtained by the ECCE partial
% deduction system . Note that the unnecessary variable Int has been
% removed (i.e. deforestation has been performed). (Also note that the
% two predicates double_app__1 and app_conj__2 are identical and could
% be merged.)
% 
% double_app__1([], X1, X2, X3) :-  app__3(X1, X2, X3).
% double_app__1([X1 | X2], X3, X4, [X1 | X5]) :-
%     app_conj__2(X2, X3, X4, X5).
% 
% app_conj__2([], X1, X2, X3) :-  app__3(X1, X2, X3).
% app_conj__2([X1 | X2], X3, X4, [X1 | X5]) :-
%     app_conj__2(X2, X3, X4, X5).
% 
% app__3([], X1, X1).
% app__3([X1 | X2], X3, [X1 | X4]) :- app__3(X2, X3, X4).
% 
% Michael Leuschel / K.U. Leuven / michael@cs.kuleuven.ac.be