[Haskell-cafe] All-fail case in asum

[David Feuer]

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" <danburton.email at gmail.com> 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 at 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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>


-- 
-- Dan Burton

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