module Main where

import Prelude

main :: IO ()
main = print $ f 0

f :: Integer -> String
f n | n > 0 = "Positive Integer"
    | n < 0 = "Negative Integer"
    | otherwise = "Zero"

Error found:
in module Main
at src/Main.hm:8:1 - 8:23 (line 8, column 1 - line 8, column 23)

  A case expression could not be determined to cover all inputs.
  The following additional cases are required to cover all inputs:

    _

  Alternatively, add a Partial constraint to the type of the enclosing value.

while applying a function $__unused
  of type Partial => t0 -> t0
  to argument case n of
                n | (greaterThan n) 0 -> "Positive Integer"
                  | (lessThan n) 0 -> "Negative Integer"
                  | otherwise -> "Zero"
while checking that expression $__unused (case n of
                                            n | (...) 0 -> "Positive Integer"
                                              | (...) 0 -> "Negative Integer"
                                              | otherwise -> "Zero"
                                         )
  has type List Char
in value declaration f

where t0 is an unknown type