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

On 10/17/2011 04:53 PM, Henk-Jan van Tuyl wrote:

On Mon, 17 Oct 2011 17:39:49 +0200, Hugo Ferreira 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