Prolog: Take the first & ldquo; N & rdquo; Elements of a list

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I need to write a Prolog predicate take(L, N, L1) which succeeds if list L1 contains the first N elements of list L, in the same order. For example:

?- take([5,1,2,7], 3, L1).
L1 = [5,1,2]
?- take([5,1,2,7], 10, L1).
L1 = [5,1,2,7]

Prolog thus far is making little sense to me, and I'm having a hard time breaking it down. Here is what I have so far:

take([H|T], 0, []).
take([H|T], N, L1) :-
   take(T, X, L2),
   X is N-1.

Can you please explain what I did wrong here?


Here is a definition that implements the relational counterpart to take in functional languages like Haskell1. First, the argument order should be different which facilitates partial application. There is a cut, but only after the error checking built-in (=<)/2 which produces an instantiation_error should the argument contain a variable.

take(N, _, Xs) :- N =< 0, !, N =:= 0, Xs = [].
take(_, [], []).
take(N, [X|Xs], [X|Ys]) :- M is N-1, take(M, Xs, Ys).

| ?- take(2, Xs, Ys).
Xs = [],
Ys = [] ? ;
Xs = [_A],
Ys = [_A] ? ;
Xs = [_A,_B|_C],
Ys = [_A,_B] ? ;
no

Note how above query reads:

How can one take 2 elements from Xs to get Ys?

And there are 3 different answers. If Xs is empty, then so is Ys. If Xs is a list with one element, then so is Ys. If Xs has at least 2 elements, then those two are Ys.


1) The only difference being that take(-1, Xs,Ys) fails (for all Xs, Ys). Probably the best would be to issue a domain_error similar to arg(-1,s(1),2)