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

On 2017-03-06 03:39 AM, Henning Thielemann wrote:

On Sun, 5 Mar 2017, 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.

A less invasive way would be to add a new function with that type.

If we're adding a new function, it might make more sense to add something more generic, like genericMany, genericSome :: (Alternative f, Applicative m, Monoid m) => f a -> f (m a) The expected type of these operations is usually fixed on the client side.

On 2017-03-07 11:40 AM, Henning Thielemann wrote:

On Tue, 7 Mar 2017, Mario Blažević wrote:

...

If we're adding a new function, it might make more sense to add something more generic, like

genericMany, genericSome :: (Alternative f, Applicative m, Monoid m) => f a -> f (m a)

The expected type of these operations is usually fixed on the client side.

I think it must be Monoid (m a). But it won't work, because NonEmpty is no Monoid because there is no mempty.

You're correct on both counts, sorry about that. It would have to be genericMany, genericSome :: (Alternative f, Applicative m, Semigroup (m a)) => f a -> f (m a) once Semigroup is in base.

2017-03-07 18:11 GMT+01:00 Henning Thielemann

:

On Tue, 7 Mar 2017, Mario Blažević wrote:

...

[...]You're correct on both counts, sorry about that. It would have to be

genericMany, genericSome :: (Alternative f, Applicative m, Semigroup (m a)) => f a -> f (m a)

once Semigroup is in base.

But then, for genericMany, Monoid would be appropriate, again. :-)

I think this discussion alone is enough of a hint that there is a need for a broader discussion to come up with coherent story regarding NonEmpty in general. ;-) Focusing on one or two functions alone will probably do more harm than improve the overall situation.

Related: https://hackage.haskell.org/package/semigroups-0.18.1/docs/src/Data.List.Non... On Wed, Mar 8, 2017 at 1:18 PM, Edward Kmett wrote:

I'm also in the "if we're going to do this, it should probably be a new function" camp. e.g. `some1`. I'm kinda neutral on adding it at all at this point, but if we do it, it should probably be done along these lines.

As for using a semigroup based implementation, its performance is, sadly, rather awful for constructing `NonEmpty` lists, and more damning, you have no law relating the 'pure' or whatever you're using to create values and the Semigroup you're using to glue them together, so it only works for NonEmpty on a purely ad-hoc type-by-type basis. That signature isn't one we want as evidenced by the fact that if you pick m = Maybe you sure as heck won't get anything like what you expect! This is the pointed problem all over again. I'm very strongly -1 on that variant of the proposal.

-Edward

On Tue, Mar 7, 2017 at 2:02 PM, Sven Panne wrote:

...

2017-03-07 18:11 GMT+01:00 Henning Thielemann < lemming@henning-thielemann.de>:

...

On Tue, 7 Mar 2017, Mario Blažević wrote:

...

[...]You're correct on both counts, sorry about that. It would have to be

genericMany, genericSome :: (Alternative f, Applicative m, Semigroup (m a)) => f a -> f (m a)

once Semigroup is in base.

But then, for genericMany, Monoid would be appropriate, again. :-)

I think this discussion alone is enough of a hint that there is a need for a broader discussion to come up with coherent story regarding NonEmpty in general. ;-) Focusing on one or two functions alone will probably do more harm than improve the overall situation.

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Ha! I saw that a while back, was pleasantly surprised, then went ahead and forgot about it. Thanks for the surprise again. Two for the price of one. On Thu, Mar 9, 2017 at 2:19 PM, Edward Kmett wrote:

Even better,

http://hackage.haskell.org/package/base-4.9.1.0/docs/ Data-List-NonEmpty.html#v:some1

is already there in base! ;)

On Tue, Mar 7, 2017 at 10:21 PM, Tony Morris wrote:

...

Related: https://hackage.haskell.org/package/semigroups-0.18.1/docs/ src/Data.List.NonEmpty.html#some1

On Wed, Mar 8, 2017 at 1:18 PM, Edward Kmett wrote:

...

I'm also in the "if we're going to do this, it should probably be a new function" camp. e.g. `some1`. I'm kinda neutral on adding it at all at this point, but if we do it, it should probably be done along these lines.

As for using a semigroup based implementation, its performance is, sadly, rather awful for constructing `NonEmpty` lists, and more damning, you have no law relating the 'pure' or whatever you're using to create values and the Semigroup you're using to glue them together, so it only works for NonEmpty on a purely ad-hoc type-by-type basis. That signature isn't one we want as evidenced by the fact that if you pick m = Maybe you sure as heck won't get anything like what you expect! This is the pointed problem all over again. I'm very strongly -1 on that variant of the proposal.

-Edward

On Tue, Mar 7, 2017 at 2:02 PM, Sven Panne wrote:

...

2017-03-07 18:11 GMT+01:00 Henning Thielemann < lemming@henning-thielemann.de>:

...

On Tue, 7 Mar 2017, Mario Blažević wrote:

...

[...]You're correct on both counts, sorry about that. It would have to be

genericMany, genericSome :: (Alternative f, Applicative m, Semigroup (m a)) => f a -> f (m a)

once Semigroup is in base.

But then, for genericMany, Monoid would be appropriate, again. :-)

I think this discussion alone is enough of a hint that there is a need for a broader discussion to come up with coherent story regarding NonEmpty in general. ;-) Focusing on one or two functions alone will probably do more harm than improve the overall situation.

_______________________________________________ Libraries mailing list Libraries@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries

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