%---------------------------------------------------------------------------%
% vim: ts=4 sw=4 et ft=mercury
%---------------------------------------------------------------------------%
%
% The "map.reduce" Benchmark
% Part of the DPPD Library.
% 
% Specialising the higher-order map/3 (using call and =..) for the
% higher-order reduce/4 in turn applied to add/3. The benchmark program
% uses built-ins but no negations. The benchmark illustrates that
% partial deduction can be used to make declarative higher-order
% programming in Prolog/LP efficient.

:- module map_reduce.

:- interface.

:- pred map_reduce is semidet.

:- implementation.

:- import_module list.
:- import_module map_impl.
:- import_module run.

map_reduce :-
    % XXX the commented-out line below results in a mode error,
    %     due to the Mercury compiler's lack of support for partially
    %     instantiated data structures.  Therefore it has been
    %     replaced with the line below it.  (The commented-out code here
    %     is also reproduced in a separate test case in tests/dppd/bug.m.)
    /* map_reduce_add([[1, 2], [1, 2, 3]], [_L1, _L2]), */
    map_reduce_add([[1, 2], [1, 2, 3]], Res0), Res0 = [_L1, _L2],
    map_reduce_add([[], [1, 2], [5, 6, 7], [8, 9, 10]], Res1),
    use(Res1),
    map_reduce_add([[], [1, 2], [5, 6, 7], [], [8, 9, 10], [11, 12],
        [13, 14], [15], [16]], Res2),
    use(Res2).

% The partial deduction query
% 
% :- map(reduce_add, L, R).
% 
% The run-time queries
% 
% :- map(reduce_add, [[1, 2], [1, 2, 3]], [L1, L2]).
% :- map(reduce_add, [[], [1, 2], [5, 6, 7], [8, 9, 10]], Res).
% :- map(reduce_add, [[], [1, 2], [5, 6, 7], [], [8, 9, 10], [11, 12],
%     [13, 14], [15], [16]], Res).
% 
% Example solution
% 
% With the ECCE partial deduction system one can obtain the following
% program (which runs more than 12 times faster than the original):
% 
% map__1([], []).
% map__1([X1 | X2], [X3 | X4]) :-
%     reduce__3(X1, X3),
%     map__1(X2, X4).
% 
% reduce__3([], 0).
% reduce__3([X1 | X2], X3) :-
%     reduce__3(X2, X4),
%     X3 is '+'(X1, X4).
% 
% Michael Leuschel / K.U. Leuven / michael@cs.kuleuven.ac.be