[Haskell-cafe] safe ways how to get head/last of Seq (or Foldable in general)
david.feuer at gmail.com
Thu Feb 26 15:03:19 UTC 2015
So are you suggesting that we add these to Data.Foldable?
headMay = foldr (\x _ -> Just x) Nothing
lastMay = foldl (\_ x -> Just x) Nothing
On Feb 26, 2015 9:18 AM, "Petr Pudlák" <petr.mvd at gmail.com> wrote:
> today I was a bit surprised that apparently there is no easy way how to
> safely get the head element of `Seq` in a point-free way. Of course there
> is `viewl`, but it seems the data type has no folding function (something
> like 'foldViewL :: b -> (a -> Seq a -> b) -> b`.
> Is there any existing function like `Seq a -> Maybe a` to safely retrieve
> the head (or last) element?
> If not, I'd suggest to add
> headMaybe :: (Foldable t) => t a -> Maybe a
> headMaybe = getFirst . foldMap (First . Just)
> and similarly lastMaybe to Data.Foldable.
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