When lists are represented by Prolog lists, a cons is represented by
the binary functor ./2 (noted [ | ])
and the empty list is represented by nil (noted []).
For instance, cons(1,cons(2,cons(3,nil))) is represented by
1.2.3.nil (noted [1,2,3]).
Prolog list representation is used in the classical list manipulation predicates: ``concatenate'', ``naïve reverse'' and ``concatenate 3 lists'':
conc([], Y, Y). conc([A|X], Y, [A|Z]) :- conc(X, Y, Z). nrev([], []). nrev([A|X], Y) :- nrev(X, RX), conc(RX, [A], Y). conc3(A, B, C, ABC) :- conc(B, C, BC), conc(A, BC, ABC).
A mode is a specification of a particular input convention.
A mode expression
is a literal in which terms are replaced by
+ (always instantiated),
- (never instantiated) and
? (do not know).
Among the three predicates above,
conc is the only one which can operate in every
mode. nrev and conc3 enter a loop for modes nrev(-,+)
and conc3(-,-,-,+). So conc can be used to split a Prolog list,
whereas conc3 cannot.
The order in which lists A,
B and C are composed in conc3 is arbitrary.
However, for every composition order there is a mode which does not work.
Note that predicate conc can concatenate a list to itself.
twice(L, LL) :- conc(L, L, LL).