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) On Jun 10, 2016 11:09 AM, "Dan Burton" wrote: Add a pattern match for the single-element list to stop it from hitting the empty list base case. asum [] = empty asum [x] = x asum (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 _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe

-- -- Dan Burton _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe