Or, to keep it a fold,
asum xs = foldr go (fromMaybe empty) xs Nothing where
go x r Nothing = r (Just x)
go x r (Just prev) = prev <|> r (Just x)
Add a pattern match for the single-element list to stop it from hitting the empty list base case.asum [] = emptyasum [x] = xasum (x:xs) = x <|> asum xs
On Friday, June 10, 2016, Tom Ellis <tom-lists-haskell-cafe-2013@jaguarpaw.co.uk> wrote:On Fri, Jun 10, 2016 at 05:37:41PM +0300, Юрий Сыровецкий (Yuriy Syrovetskiy) wrote:
> It is so because in base library
>
> asum = foldr (<|>) empty
>
> What if it was defined
>
> asum [] = empty
> asum [x:xs] = x <|> asum xs
Those are the same (assuming you mean (x:xs) instead of [x:xs]).
Tom
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