Re: Alternative.some and NonEmpty

that's solvable with CPP if needed

Note that it's possible to avoid CPP because one can use `fmap toList . some` to achieve the old behavior. And the breakage is not silent, so it won't be too hard for maintainers. On Sun, Mar 5, 2017 at 2:49 PM, Ivan Lazar Miljenovic wrote:

On 5 March 2017 at 21:51, Vladislav Zavialov wrote:

...

Since 'Data.List.NonEmpty' is now in 'base', it makes sense to change the type of 'some' from

some :: Alternative f => f a -> f [a]

to

some :: Alternative f => f a -> f (NonEmpty a)

as it's guaranteed to return a non-empty list.

Currently, both users and implementors of 'Alternative' instances are at a disadvantage. The users have to use the unsafe NonEmpty.fromList function to take advantage of the fact that the result of 'some' is indeed non-empty. The implementors have more space for error - it's possible to accidentally return an empty list.

The disadvantage is also for people trying to maintain a code base, though that's solvable with CPP if needed.

However, this also affects the default instances.

(As a user and implementor of Alternative instances, I've never found myself at a disadvantage for using the current type.)

...

I volunteer to implement. Does everyone agree it's a good idea?

I'm -0.5.

-- Ivan Lazar Miljenovic Ivan.Miljenovic@gmail.com http://IvanMiljenovic.wordpress.com