Either and explanatory failures

The idea

Maybe represents absence, while Either can also carry an explanation. A result such as Either String Integer makes both the error information and successful value visible in the type. Pattern matching forces callers to consider the alternatives instead of treating a special integer as a failure sentinel that might collide with valid data.

Dependent checked computations can be sequenced so that a Left propagates and later successful work is skipped. This expresses a failure policy rather than throwing a hidden exception from an otherwise ordinary-looking function. Keep error messages or error constructors meaningful enough for the layer that handles them. A total interface should also account for input boundaries such as a zero divisor before performing the unsafe operation.

divide :: Integer -> Integer -> Either String Integer
divide _ 0 = Left "zero divisor"
divide a b = Right (a `div` b)
main = do
  print (divide 9 2)
  print (divide 9 0)

Right 4
Left "zero divisor"

The zero case is handled before division, and the successful branch contains its integer quotient explicitly.