[Haskell-beginners] Non exhaustive pattern match not flagged?

[Hugo Ferreira]

On 10/17/2011 04:53 PM, Henk-Jan van Tuyl wrote:
> On Mon, 17 Oct 2011 17:39:49 +0200, Hugo Ferreira <hmf at inescporto.pt>
> wrote:
>
>> f :: [a] -> [b] -> Int
>> --f [] _ = error "undefined for empty array"
>> f _ [] = error "undefined for empty array"
>> f (_:xs) (_:ys) = length xs + length ys
>>
>> I get a warning:
>>
>> Warning: Pattern match(es) are non-exhaustive
>> In an equation for `f': Patterns not matched: [] (_ : _)
>>
>> as expected. But for:
>>
>> bigrams :: [a] -> [[a]]
>> --bigrams [] = []
>> bigrams [_] = []
>> bigrams xs = take 2 xs : bigrams (tail xs)
>>
>> I don't. Why is the first predicate not detected as missing?
>
> The pattern in the line
> bigrams xs = take 2 xs : bigrams (tail xs)
> matches any list, even empty lists
>

Thank you.
Hugo F.

> Regards,
> Henk-Jan van Tuyl
>
>