Can't find the infinite loop in my Haskell code

Question

I'm learning myself Haskell, and for one exercise I have to calculate the determinant for a Matrix. I refreshed the mathematical way to do it and emulated it in code, but for some reason it's looping infinitely. I've been entering stuff into GHCi for hours trying to pinpoint what's causing it but I can't find it.

type Vector = [Float]
type Matrix = [Vector]

determinant :: Matrix -> Float
determinant [] = 0
determinant [[a,b],[c,d]] = (a*d) - (b*c)
determinant (vec:mat) = dethelper vec mat 0

dethelper :: Vector -> Matrix -> Int -> Float
dethelper vec mat n
  | vec == []              = 0
  | n == length vec        = 0 
  | even n                 = (vec!!n * (determinant $ map (dropAt n) mat)) - (dethelper vec mat n+1)
  | otherwise              = (vec!!n * (determinant $ map (dropAt n) mat)) + (dethelper vec mat n+1)

dropAt :: Int -> Vector -> Vector
dropAt x xs = (fst spl) ++ (tail $ snd spl)
               where spl = splitAt x xs

How do you invoke it? Have you tested each one of the functions? — Eugene Sh.
– Eugene Sh., Commented May 5, 2016 at 21:18
dethelper vec mat n+1 means (dethelper vec mat n)+1 not dethelper vec mat (n+1), which is presumably what you wanted. — user2407038
– user2407038, Commented May 5, 2016 at 21:52

NovaDenizen · Accepted Answer · 2016-05-06 21:58:21Z

1

dethelper vec mat n+1 does not work how you think it does. It is parsed like (dethelper vec mat n)+1. I think you actually want dethelper vec mat (n+1)

answered May 6, 2016 at 21:58

NovaDenizen

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