Incomplete patterns [GHC-62161]

Flag: -Wincomplete-patterns
Enabled by: -Wextra

A function definition or case expression did not cover all cases. This may result in a run-time crash of the program.

Examples

Sum of a list

The case for the empty list is missing.

Error Message

Main.hs:4:1: warning: [-Wincomplete-patterns] [GHC-62161]
    Pattern match(es) are non-exhaustive
    In an equation for ‘mySum’:
        Patterns of type ‘[Int]’ not matched: []
  |
2 | mySum (x : xs) = x + mySum xs
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Main.hs

Before

{-# OPTIONS_GHC -Wincomplete-patterns #-}

mySum :: [Int] -> Int
mySum (x : xs) = x + mySum xs

After

{-# OPTIONS_GHC -Wincomplete-patterns #-}

mySum :: [Int] -> Int
mySum (x : xs) = x + mySum xs
mySum [] = 0

Safe head of a list

The case for the empty list is missing, and for a safe version of head, the return type has to be changed.

Error Message

Main.hs:4:1: warning: [-Wincomplete-patterns]
    Pattern match(es) are non-exhaustive
    In an equation for ‘safeHead’:
        Patterns of type ‘[a]’ not matched: []
  |
3 | safeHead (x : xs) = x
  | ^^^^^^^^^^^^^^^^^^^^^

Main.hs

Before

{-# OPTIONS_GHC -Wincomplete-patterns #-}

safeHead :: [a] -> a
safeHead (x : xs) = x

After

{-# OPTIONS_GHC -Wincomplete-patterns #-}

safeHead :: [a] -> Maybe a
safeHead (x : xs) = Just x
safeHead [] = Nothing