Question

I am trying to write a Tail Recursive procedure to count the number of uninstantiated variables in a list. I am a little stuck, where am I going wrong.

My current query is below:

count([S,L],N) :- var(S), !, N+1.
count([L],N).

Answer 1

Note: this answer presents a solution that is recursive but not tail recursive. For a tail recursive solution you should use an accumulator as can be shown in other answers from this question.

As with any recursive procedure, you should add a proper base case.
In this case, it should be a clause with an empty list that returns unifies 0 with the number of uninstantiated variables:

count([], 0).

Check the clause you’ve written. It takes as input a list of two elements instead of a list represented as a Head item and a tail list, and it really does nothing with N:

count([Head|Tail], M):- 
    var(Head), 
    !, 
    count(Tail, N), 
    M is N+1.

And finally, you should also add a clause to deal with the case when the first item of the list is not an uninstantiated variable:

count([_|Tail], N):- count(Tail, N).