Why does dialyzer find my type specification invalid?

2
votes

I am new to dialyzer, and I am hoping someone could give me a quick idea of it's operation by answering this question.

I would think that the following function, given a number X and a non-negative integer N, would produce a number. (X to the Nth power.)

-spec pow(X :: number(), N :: non_neg_integer) -> number().
pow(X, N) ->
    pow(X, N, 1).
pow(_X, 0, R) ->
    R;
pow(X, N, R) ->
    pow(X*X,
        N bsr 1,
        if
            N band 1 =:= 0 ->
                R;
            true ->
                X*R
        end).

But dialyzer doesn't like my specification. It tells me:

Invalid type specification for function t:pow/2.
The success typing is (_,integer()) -> any()

What it suggests seems to me to be an overly inclusive specification. Can someone explain why it does this, and whether there is any way to get a more restrictive type specification accepted?

1
erlangtype-systemsdialyzer

1 Answers

8
votes

I believe it is because of you wrote non_neg_integer instead of non_neg_integer().