Misleading strictness annotations in Data.List.NonEmpty

[Viktor Dukhovni]

On Fri, Jan 08, 2021 at 09:44:31PM +0200, Oleg Grenrus wrote:
> Note also
> 
> -- | @since 4.9.0.0
> instance Foldable NonEmpty where
>   foldr f z ~(a :| as) = f a (List.foldr f z as)
>   foldl f z (a :| as) = List.foldl f (f z a) as
>   foldl1 f (a :| as) = List.foldl f a as
> 
>   -- GHC isn't clever enough to transform the default definition
>   -- into anything like this, so we'd end up shuffling a bunch of
>   -- Maybes around.
>   foldr1 f (p :| ps) = foldr go id ps p
>     where
>       go x r prev = f prev (r x)
> 
>   -- We used to say
>   --
>   --   length (_ :| as) = 1 + length as
>   --
>   -- but the default definition is better, counting from 1.
>   --
>   -- The default definition also works great for null and foldl'.
>   -- As usual for cons lists, foldr' is basically hopeless.
> 
>   foldMap f ~(a :| as) = f a `mappend` foldMap f as
>   fold ~(m :| ms) = m `mappend` fold ms
>   toList ~(a :| as) = a : as
> 
> Plenty of irrefutable patterns.

Do any of these make "mfix" more usable for NonEmpty?  Or are they just
superfluous?  With just one constructor, is there any downside to an
irrefutable pattern?

-- 
    Viktor.