Adding integers in list

For so开发者_如何学运维me reason, this is not working. I am getting: ERROR: is/2: Arguments are not sufficiently instantiated

1 add_list([]).
2 add_list([H|T]):-
3                 Sum2 is Sum1 + H,
4                 add_list(T).

I am trying to add the contents of a list (containing only numbers).

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I'm not sure what you are trying to do. But if you are trying to calc total sum it will be this way (changed name to list_sum as add_list doesn't make any sense):

list_sum([], 0).
list_sum([H|T], Sum):-
    list_sum(T, SubSum),
    Sum is SubSum + H.

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You can have a "functionnal mind" with foldl :

foldl(_P, [], V, V).

foldl(P, [H|T], V1, VF) :-
    call(P, H, V1, V2),
    foldl(P, T, V2, VF).


sum_list(L, S) :-
    foldl(add, L, 0, S).


add(X, Y, Z) :-
    Z is X+Y.

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Alternatively you could also use an accumulator (the advantage is, that it is tail-recursive and therefore can be optimized)

list_sum(L,R) :- list_sum(L,0,R).   
list_sum([],A,A). 
list_sum([H|T],A,R) :- A1 is A + H, list_sum(T,A1,R).