I'm relatively new to Haskell and rather inexperienced with programming in general. A function is causing a stack overflow on certain inputs and I'm not sure why.

Function in question:

digitSum :: Integer -> Integer -> Integer
digitSum _ 1 = 1
digitSum base x = (x `mod` base)   digitSum base (x `div`base)

On some inputs (e.g. 10 15, 11 121, 16 19, 3 1234) it works while on others (10 456 for example) it breaks Could someone explain this to me, so that I can avoid it in the future?

CodePudding user response：

Stack overflow in a functional language often means: unbounded recursion. And, indeed, it is what happens here, for a lot of inputs: take for instance digitSum 2 0. It doesn't match the first pattern match, so it goes for the second case, with base=2 and x=0. x `mod` base = 1 and x `div` base = 0. It then makes a recursive call to digitSum 2 0. Then forever.

This is because your base case is wrong: it should be digitSum _ 0 = 0 instead, because if you keep integer dividing x by base, if base is not 1, you'll always end up with 0.