GHC compiler warnings

dig.hs

sumDigits :: [Int] -> Int
sumDigits (x:xs) = x + sumDigits xs
-- Missing the "sumDigits [] = 0" base case.

main = do
  putStr $ show $ sumDigits [1,2,3]

ghc

# Without options:
$ ghc
Linking dig ...

# With -Wall:
$ ghc dig.hs -Wall
[1 of 1] Compiling Main             ( dig.hs, dig.o )

dig.hs:4:1: Warning:
    Top-level binding with no type signature: main :: IO ()

dig.hs:2:1: Warning:
    Pattern match(es) are non-exhaustive
    In an equation for ‘sumDigits’: Patterns not matched: []
Linking dig ...