56 lines
1.4 KiB
Prolog
56 lines
1.4 KiB
Prolog
/** Reasoning about pairs.
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Pairs are Prolog terms with principal functor `(-)/2`. A pair
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often has the form `Key-Value`. The predicates of this library
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relate pairs to keys and values.
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*/
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:- module(pairs, [pairs_keys_values/3,
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pairs_keys/2,
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pairs_values/2,
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group_pairs_by_key/2,
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map_list_to_pairs/3]).
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:- meta_predicate(map_list_to_pairs(2, ?, ?)).
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%% pairs_keys_values(?Pairs, ?Keys, ?Values)
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%
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% The first argument is a list of Pairs, the second the corresponding
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% Keys, and the third argument the corresponding values.
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pairs_keys_values([], [], []).
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pairs_keys_values([A-B|ABs], [A|As], [B|Bs]) :-
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pairs_keys_values(ABs, As, Bs).
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%% pairs_keys(?Pairs, ?Keys)
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%
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% Same as `pairs_keys_values(Pairs, Keys, _)`.
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pairs_keys(Ps, Ks) :- pairs_keys_values(Ps, Ks, _).
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%% pairs_values(?Pairs, ?Values)
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%
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% Same as `pairs_keys_values(Pairs, _, Values)`.
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pairs_values(Ps, Vs) :- pairs_keys_values(Ps, _, Vs).
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map_list_to_pairs(Pred, Ls, Ps) :-
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map_list_to_pairs2(Ls, Pred, Ps).
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map_list_to_pairs2([], _, []).
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map_list_to_pairs2([H|T0], Pred, [K-H|T]) :-
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call(Pred, H, K),
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map_list_to_pairs2(T0, Pred, T).
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group_pairs_by_key([], []).
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group_pairs_by_key([K-V|KVs0], [K-[V|Vs]|KVs]) :-
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same_key(K, KVs0, Vs, KVs1),
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group_pairs_by_key(KVs1, KVs).
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same_key(K0, [K1-V|KVs0], [V|Vs], KVs) :-
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K0 == K1, !,
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same_key(K0, KVs0, Vs, KVs).
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same_key(_, KVs, [], KVs).
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