Patterns and guards

The idea

Enter with function signatures and application understood. Pattern matching decomposes values by their structure. The pattern [] matches an empty list, while x:xs matches a nonempty list with head x and remaining tail xs. Function equations are tried in order, so an earlier general pattern can make a later special case unreachable. Cover the cases that the function’s advertised inputs allow.

Guards refine an equation using boolean conditions. They are checked in order, and otherwise supplies a conventional final true condition. Keep a guard focused on a property of already matched values instead of forcing every decision into nested conditionals. A complete function handles boundaries deliberately, including zero and empty input when applicable. Missing cases can fail at runtime when the uncovered value is demanded.

label :: Integer -> String
label n
  | n < 0 = "negative"
  | n == 0 = "zero"
  | otherwise = "positive"
main = putStrLn (label 0)

zero

The negative guard fails, the equality guard succeeds, and the remaining guard is not selected.