[Haskell-beginners] Question re "Pattern match(es) are non-exhaustive"

[Mike Meyer]

On Tue, Feb 10, 2015 at 6:58 AM, Joel Neely <joel.neely at gmail.com> wrote:

> {-# OPTIONS_GHC -Wall #-}
>
> sumDigits :: [Integer] -> Integer
> sumDigits []     = 0
> sumDigits (n:ns)
>   | n < 10       = n + sumDigits ns
>   | n >= 10      = r + sumDigits (q : ns)
>   where (q, r)   = n `quotRem` 10
>
> sumList :: [Integer] -> Integer
> sumList []     = 0
> sumList (n:ns) = n + sumList ns
>
>
>
ghc can't decide whether or not the tests in the guards are complete or
not. If you rewrite the second condition as "otherwise", it'll recognize
the catch-all and you won't get a warning.
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