Haskell - Strange truncation behavior of `div`

Question

In ghci I get the following:

λ> -1 `div` 2
0

However:

λ> map (`div` 2) [-1]
[-1]

The problem arose when I was using a function divPair:

divPair :: (Int, Int) -> Int -> (Int, Int)
divPair (a, b) n = (a `div` n, b `div` n)

λ> divPair (-1, -2) 2
(-1,-1)

which was really a surprise to me as I expected it to produce (0, -1).

What happened here? I suppose I didn't really understand something about div in Haskell.

Answer 1

That's because -1 `div` 2 is actually parsed as -(1 `div` 2) which equals -0 or just 0 as 1 `div` 2 equals 0. You should use (-1) `div` 2.

Prelude> -(1 `div` 2)
0
Prelude> (-1) `div` 2
-1