It is folk knowledge that append(X,[Y],Z)
finds the last element
Y
of the list Z
and the remaining list X
.
But there is some advantage of having a customized predicate last/3
,
namely it can react without leaving a choice point:
?- last([1,2,3],X,Y).
X = 3,
Y = [1,2]
?- append(Y,[X],[1,2,3]).
Y = [1,2],
X = 3 ;
No
Is there a way to realize a different implementation of
append/3
which would also not leave a choice point in the
above example?
P.S.: I am comparing:
/**
* append(L1, L2, L3):
* The predicate succeeds whenever L3 unifies with the concatenation of L1 and L2.
*/
% append(+List, +List, -List)
:- public append/3.
append([], X, X).
append([X|Y], Z, [X|T]) :- append(Y, Z, T).
And (à la Gertjan van Noord):
/**
* last(L, E, R):
* The predicate succeeds with E being the last element of the list L
* and R being the remainder of the list.
*/
% last(+List, -Elem, -List)
:- public last/3.
last([X|Y], Z, T) :- last2(Y, X, Z, T).
% last2(+List, +Elem, -Elem, -List)
:- private last2/4.
last2([], X, X, []).
last2([X|Y], U, Z, [U|T]) :- last2(Y, X, Z, T).
One way to do it is to use
foldl/4
with the appropriate help predicate:This should be it:
Try to see if you can leave out the
is_list/1
from the definition.