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

Petr Pudlák petr.mvd at gmail.com
Thu Feb 26 14:18:20 UTC 2015


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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