[Haskell-cafe] Re: snd and tuples of various sizes...

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"Mattias" == Mattias Bengtsson writes:

Mattias> On Thu, 2007-02-01 at 21:01 -1000, Tim Newsham wrote:

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instance Second [a] a where snd [] = error "don't got none" snd (x:y:xs) = y

Mattias> Would'nt that instance mean this: snd [] produces error Mattias> snd [x] gives [] No. It is non-exhaustive pattern. In fact this is needed:

instance Second [a] a where snd (_:y:_) = y snd _ = error "don't got none"

Mattias> I'd implement it something like this (if this works?): Mattias> instance Second [a] (Maybe a) where snd [] = Nothing snd Mattias> [x] = Nothing snd (x:y:xs) = Just y Well, we also can define:

class SafeSecond a b | a -> b where ssnd :: (Monad m) => a -> m b

instance SafeSecond [a] a where ssnd (_:y:_) = return y ssnd _ = fail "don't got none"

main = do print $ (ssnd [1, 2, 3] :: Maybe Int) print $ (ssnd [1] :: Maybe Int)

-- WBR, Max Vasin.