When is the difference between quotRem and divMod useful?

Question

From the haskell report:

The quot, rem, div, and mod class methods satisfy these laws if y is non-zero:

(x `quot` y)*y + (x `rem`          


        

Answer 1

A simple example where it would matter is testing if an integer is even or odd.

let buggyOdd x = x `rem` 2 == 1
buggyOdd 1 // True
buggyOdd (-1) // False (wrong!)

let odd x = x `mod` 2 == 1
odd 1 // True
odd (-1) // True

Note, of course, you could avoid thinking about these issues by just defining odd in this way:

let odd x = x `rem` 2 /= 0
odd 1 // True
odd (-1) // True

In general, just remember that, for y > 0, x mod y always return something >= 0 while x rem y returns 0 or something of the same sign as x.