[Haskell-cafe] safe ways how to get head/last of Seq (or Foldable in general)

[Michael Snoyman]

The mono-traversable package provides headMay[1] which works for Seq.

[1]
http://www.stackage.org/haddock/nightly-2015-02-25/mono-traversable-0.9.0.1/Data-MonoTraversable.html#v:headMay

On Thu Feb 26 2015 at 4:18:31 PM Petr Pudlák <petr.mvd at gmail.com> wrote:

> Hi,
>
> 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.
>
>   Thanks,
>   Petr
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