roman-numerals: test for Nothing

exercises/practice/roman-numerals/.meta/examples/success-standard/src/Roman.hs

@@ -1,7 +1,8 @@
 module Roman (numerals) where
 
 numerals :: Int -> Maybe String
-numerals = Just . go numeralMap
+numerals number | number < 1 || number > 3999 = Nothing
+numerals number = Just $ go numeralMap number
   where
     go pairs@((value, digits):pairs') n
       | n >= value = digits ++ go pairs (n - value)

exercises/practice/roman-numerals/test/Tests.hs

@@ -101,4 +101,16 @@ cases = [ Case { description = "1 is a single I"
                , number      = 3000
                , expected    = Just "MMM"
                }
+        , Case { description = "4000 cannot be represented"
+               , number      = 4000
+               , expected    = Nothing
+               }
+        , Case { description = "0 cannot be represented"
+               , number      = 0
+               , expected    = Nothing
+               }
+        , Case { description = "-1 cannot be represented"
+               , number      = -1
+               , expected    = Nothing
+               }
         ]