Question

Hello stackoverflow community,

I have a rather simple piece of code that is getting a strange error. Here is my code:

multiplyByTwo 0 = 1
myltiplyByTwo x = 2 * multiplyByTwo (x-1)

as you can see, it is simple recursion. I am getting the “Non-exhaustive pattern in function
multiplyByTwo” exception. I do not understand why I am getting this, as I have already set the lowest value multiplyByTwo 0 = 1 therefore I would guess that the function is not calling itself after that point. What I am saying is that I do not understand why this pattern is “non-exhaustive”. I am assuming that the number you enter for x is positive.

thank you for any help you can give.

Answer 1

You have a typo in the second line:

myltiplyByTwo x = 2 * multiplyByTwo (x-1)
 ^

thus that defines a new function, and multiplyByTwo is only defined for the argument 0.