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 Haskell^{1}. 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)}