Ok, Foldable is a formal condition for Traversable, but not actually used in the implementation of Traversable Either. This still leaves room to implement Foldable for Either by instance Foldable (Either a) where foldMap _ _ = error "Folding Either? Naah, I don't think this is a good idea." On 02.03.2017 17:48, David Feuer wrote:

The problem is that we'd then lose the perfectly good Traversable instance, which would be sad.

On Mar 2, 2017 11:23 AM, "Andreas Abel" <andreas.abel@ifi.lmu.de <mailto:andreas.abel@ifi.lmu.de>> wrote:

Today a student came to me wondering why a certain function produced a regular result, where he had expected an error. Turned out he had used `concat`, but not on a lists of lists as he had thought, but on a lists of `Either a [b]`.

With the Foldable instance for Either, which considers Either a b to be a container of 0-1 elements of b, errors are happily swallowed.

I think this instance is harmful and should be deprecated (and later removed) from base.

There are similarly pointless Foldable instances as well.

See a discussion one year ago, which was heated, but had no consequences.

https://mail.haskell.org/pipermail/libraries/2016-February/026678.html <https://mail.haskell.org/pipermail/libraries/2016-February/026678.html>

-- Andreas Abel <>< Du bist der geliebte Mensch.

Department of Computer Science and Engineering Chalmers and Gothenburg University, Sweden

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-- Andreas Abel <>< Du bist der geliebte Mensch. Department of Computer Science and Engineering Chalmers and Gothenburg University, Sweden andreas.abel@gu.se http://www.cse.chalmers.se/~abela/

Personally, I think it would be a shame to lose foldMap on EIther. I frequently foldMap over Maybe values (where mempty is suitable in case of "failure"), and I can certainly see myself doing the same thing with Either. - ocharles On Thu, Mar 2, 2017 at 5:08 PM Francesco Ariis <fa-ml@ariis.it> wrote:

On Thu, Mar 02, 2017 at 05:59:18PM +0100, Andreas Abel wrote:

...

Ok, Foldable is a formal condition for Traversable, but not actually used in the implementation of Traversable Either. This still leaves room to implement Foldable for Either by

instance Foldable (Either a) where foldMap _ _ = error "Folding Either? Naah, I don't think this is a good idea."

I don't fancy Foldable for Either either, but runtime failures are ugly too.

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As with all of these discussions, the point of having Either be Foldable is not that you're going to call foldMap on an Either value directly. Instead, it is that you be able to pass whatever sort of foldable thing you choose (which Either certainly is) to a function that only requires foldability of its input. By removing or damaging the Foldable instance for Either, you don't simply prevent people from encountering problems that will be resolved the first time they test their software - you make a whole universe of nicely polymorphic functions unavailable for them to use without additional effort. In particular, the idea that one should make all such functions partial by throwing an error is repugnant. Kris On Thu, Mar 2, 2017 at 10:35 AM, Francesco Ariis <fa-ml@ariis.it> wrote:

On Thu, Mar 02, 2017 at 05:19:26PM +0000, Oliver Charles wrote:

...

Personally, I think it would be a shame to lose foldMap on EIther. I frequently foldMap over Maybe values (where mempty is suitable in case of "failure"), and I can certainly see myself doing the same thing with Either.

I am not trying to be polemic, just to see where the community stands: regarding Either/Maybe: do you have use cases for length (sum) too?

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foldMapDefault and fmapDefault *should* actually perform quite well, by the way, unless the specialized definitions come with rewrite rules. On Mar 2, 2017 1:50 PM, "Dan Burton" <danburton.email@gmail.com> wrote:

Of course errors are swallowed. Look at the type signature.

concat :: Foldable t => t [b] -> [b] -- t = Either a concat :: Either a [b] -> [b]

The error type "a" does not appear anywhere in the function's output type. In the event of an error, the only thing it could possibly produce as output is an empty list.

I don't think this means that Either's foldable instance is harmful and should be removed. It's a perfectly good instance that behaves in the "obvious" Maybe-like way. I do think this is an argument in favor of using a custom Prelude (which uses functions specialized to lists) when teaching new students.

Any Traversable instance should absolutely be Foldable, with foldMap = foldMapDefault, or an optimized version that produces the same result. Putting `foldMap = error ...` for any Traversable is out of the question, imo.

-- Dan Burton

On Thu, Mar 2, 2017 at 10:12 AM, Kris Nuttycombe < kris.nuttycombe@gmail.com> wrote:

...

As with all of these discussions, the point of having Either be Foldable is not that you're going to call foldMap on an Either value directly. Instead, it is that you be able to pass whatever sort of foldable thing you choose (which Either certainly is) to a function that only requires foldability of its input. By removing or damaging the Foldable instance for Either, you don't simply prevent people from encountering problems that will be resolved the first time they test their software - you make a whole universe of nicely polymorphic functions unavailable for them to use without additional effort.

In particular, the idea that one should make all such functions partial by throwing an error is repugnant.

Kris

On Thu, Mar 2, 2017 at 10:35 AM, Francesco Ariis <fa-ml@ariis.it> wrote:

...

On Thu, Mar 02, 2017 at 05:19:26PM +0000, Oliver Charles wrote:

...

Personally, I think it would be a shame to lose foldMap on EIther. I frequently foldMap over Maybe values (where mempty is suitable in case of "failure"), and I can certainly see myself doing the same thing with Either.

I am not trying to be polemic, just to see where the community stands: regarding Either/Maybe: do you have use cases for length (sum) too?

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Hi, Francesco Ariis wrote:

I am not trying to be polemic, just to see where the community stands:

Well, personally, I would not miss neither Foldable nor Traversable instances of (Either b) or ((,) b) for even a fraction of a second. And I know many people who fully agrees. Henning summed it up very well, I think: https://mail.haskell.org/pipermail/libraries/2016-February/026678.html I am completely and utterly unconvinced by arguments along the lines that just because one of the standard types *can* be seen as a container type it must be seen that way, by everyone, no exceptions. That was never the intent of these types, as evidenced by their names and as evidenced by the inherent asymmetry of their instances. Pretending otherwise just creates confusion, of which there are plenty of instances, with more coming as per Andreas' initial mail. Further this generality buys very little if anything in practice as anyone who really needs the functionality can define their own instances or, better, introduce a custom type with a well-chosen name that clearly reflects the application in question. So, indeed, "all consistent but consistently useless", as Henning eloquently put it. To be concrete: after all, why would I write, for an x of type Either Int, say, foldl (+) 0 x and not simply either (const 0) id x ? I know what I find more readable, at least, and I'd be willing to bet on what is more maintainable as well as time passes and code is passed from one group of maintainers to another. Yes, I did see: Kris Nuttycombe wrote:

As with all of these discussions, the point of having Either be Foldable is not that you're going to call foldMap on an Either value directly.

But I must say that strikes me as supremely odd: why on Earth should one *not* make direct use of instance methods if they make so much sense as is claimed? Well, I don't really expect any changes. This is just to say that there still are lots of people who find the push to instantiate Foldable and Traversable as widely as possible very unfortunate. Best, /Henrik This message and any attachment are intended solely for the addressee and may contain confidential information. If you have received this message in error, please send it back to me, and immediately delete it. Please do not use, copy or disclose the information contained in this message or in any attachment. Any views or opinions expressed by the author of this email do not necessarily reflect the views of the University of Nottingham. This message has been checked for viruses but the contents of an attachment may still contain software viruses which could damage your computer system, you are advised to perform your own checks. Email communications with the University of Nottingham may be monitored as permitted by UK legislation.

100% this. Cofree Maybe is a a nicely general encoding of a non-empty list that shows how closely related it is to a rose tree Cofree []. It'd lose its ability to be folded if Maybe ceased to be Foldable. When I'm showing how the cofree comonad works, I often take time to work up those examples along with Cofree ((->) e) as an (infinite) Moore machine. About 3-4 examples in, it usually clicks. instances on Either e, and Maybe, Identity, (,) e, etc. are all building blocks that are commonly used for larger things like this. As you enumerate all the simple functors we have lying around in Haskell, looking at what they do when you compute `Cofree f` or `Free f` tends to be quite informative. This teaching approach breaks down completely once you start cutting arbitrary holes in instance coverage and making everyone remember where those holes are. -Edward On Fri, Mar 3, 2017 at 5:04 PM, David Menendez <dave@zednenem.com> wrote:

On Thu, Mar 2, 2017 at 1:12 PM, Kris Nuttycombe <kris.nuttycombe@gmail.com

...

wrote:

...

As with all of these discussions, the point of having Either be Foldable is not that you're going to call foldMap on an Either value directly. Instead, it is that you be able to pass whatever sort of foldable thing you choose (which Either certainly is) to a function that only requires foldability of its input. By removing or damaging the Foldable instance for Either, you don't simply prevent people from encountering problems that will be resolved the first time they test their software - you make a whole universe of nicely polymorphic functions unavailable for them to use without additional effort.

Not just polymorphic functions. Many higher-order type constructors have Foldable instances based on their parameters. For example,

data Cofree f a = a :< f (Cofree f a)

instance Foldable f => Foldable (Cofree f a) where foldMap f (a :< as) = f a <> foldMap (foldMap f) as

Without the instance for Either b, we lose the instances for Cofree (Either b) and Cofree (Compose [] (Either b)), both of which seem reasonable and useful.

In particular, the idea that one should make all such functions partial by

...

throwing an error is repugnant.

Yes, that seems like the worst possible solution. Better would be some way to give a warning when calling an overloaded function at a particular type.

-- Dave Menendez <dave@zednenem.com> <http://www.eyrie.org/~zednenem/>

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It would thrash the entire community back more than half-way to where things stood before the Foldable/Traversable proposal and force us back into a world of qualified imports. Everyone who has adapted their code to the status quo over the last 2-3 years and have enjoyed the benefit of a base that doesn't have any internal conflicts would have to go through all the work of reverting their code, or add yet more CPP to hack around the mess. That strikes me, on balance, as a bad option. -Edward On Sat, Mar 4, 2017 at 9:16 PM, <amindfv@gmail.com> wrote:

To Richard's point, though, the ability right now to have e.g. Cofree (Compose [] (Either b)) is in direct conflict with not allowing much simpler terms that no one would ever want to call, e.g.:

sum ([1,2,3], 1) == 1

At the moment, we're picking points on the expressiveness-correctness spectrum.

Would David's proposal of making Foldable a qualified import not allow us to have our cake and eat it too?

Tom

El 3 mar 2017, a las 23:10, Edward Kmett <ekmett@gmail.com> escribió:

100% this. Cofree Maybe is a a nicely general encoding of a non-empty list that shows how closely related it is to a rose tree Cofree []. It'd lose its ability to be folded if Maybe ceased to be Foldable.

When I'm showing how the cofree comonad works, I often take time to work up those examples along with Cofree ((->) e) as an (infinite) Moore machine. About 3-4 examples in, it usually clicks.

instances on Either e, and Maybe, Identity, (,) e, etc. are all building blocks that are commonly used for larger things like this.

As you enumerate all the simple functors we have lying around in Haskell, looking at what they do when you compute `Cofree f` or `Free f` tends to be quite informative.

This teaching approach breaks down completely once you start cutting arbitrary holes in instance coverage and making everyone remember where those holes are.

-Edward

On Fri, Mar 3, 2017 at 5:04 PM, David Menendez <dave@zednenem.com> wrote:

...

On Thu, Mar 2, 2017 at 1:12 PM, Kris Nuttycombe < kris.nuttycombe@gmail.com> wrote:

...

As with all of these discussions, the point of having Either be Foldable is not that you're going to call foldMap on an Either value directly. Instead, it is that you be able to pass whatever sort of foldable thing you choose (which Either certainly is) to a function that only requires foldability of its input. By removing or damaging the Foldable instance for Either, you don't simply prevent people from encountering problems that will be resolved the first time they test their software - you make a whole universe of nicely polymorphic functions unavailable for them to use without additional effort.

Not just polymorphic functions. Many higher-order type constructors have Foldable instances based on their parameters. For example,

data Cofree f a = a :< f (Cofree f a)

instance Foldable f => Foldable (Cofree f a) where foldMap f (a :< as) = f a <> foldMap (foldMap f) as

Without the instance for Either b, we lose the instances for Cofree (Either b) and Cofree (Compose [] (Either b)), both of which seem reasonable and useful.

In particular, the idea that one should make all such functions partial

...

by throwing an error is repugnant.

Yes, that seems like the worst possible solution. Better would be some way to give a warning when calling an overloaded function at a particular type.

-- Dave Menendez <dave@zednenem.com> <http://www.eyrie.org/~zednenem/>

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We had a very similar discussion this time last year ( https://mail.haskell.org/pipermail/libraries/2016-February/026678.html), with many people on both sides of the issue. There wasn't enough agreement reached to change the status quo. I'll just repeat two things from that discussion: - This is *not* just an issue of "beginners may be confused": Andrew Farmer describes encountering what I think is the most disconcerting part of having these instances: you can no longer have confidence in the process of change-the-type-then-just-fix-the-type-errors: https://mail.haskell.org/pipermail/libraries/2016-February/026782.html - The tl;dr of my argument last time: "Part of the power of a type system is which program it rejects, and I'm arguing we're cluttering the space of valid programs." The last time this was discussed, we planned to implement a flag to forbid certain instances. Has anyone taken that up? Tom On Thu, Mar 2, 2017 at 5:35 PM, Oleg Grenrus <oleg.grenrus@iki.fi> wrote:

Anecdotally, today I wrote `foldMap toList`, to use as `Maybe (NonEmpty a) -> [a]`. Also I rely heavily in lens-based code on the `folded :: Fold (Maybe a) a`.

So :-1: for me.

OTOH :+1: for using non-default preludes for teaching.

- Oleg

On 02.03.2017 19:35, Francesco Ariis wrote:

...

On Thu, Mar 02, 2017 at 05:19:26PM +0000, Oliver Charles wrote:

...

Personally, I think it would be a shame to lose foldMap on EIther. I frequently foldMap over Maybe values (where mempty is suitable in case of "failure"), and I can certainly see myself doing the same thing with Either. I am not trying to be polemic, just to see where the community stands: regarding Either/Maybe: do you have use cases for length (sum) too?

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We could have a module Data.List.ReallyJustListsAndNotSomeThingMoreGeneric which implements concat and friends just for lists and could be imported if one wants to have the list operations. Currently, import qualified Data.List as List does not give on the list operations as e.g. List.concat but the generic ones. See also http://ghc.haskell.org/trac/ghc/ticket/13345 On 02.03.2017 20:02, Edward Kmett wrote:

On Thu, Mar 2, 2017 at 11:59 AM, Andreas Abel <andreas.abel@ifi.lmu.de <mailto:andreas.abel@ifi.lmu.de>> wrote:

Ok, Foldable is a formal condition for Traversable, but not actually used in the implementation of Traversable Either. This still leaves room to implement Foldable for Either by

instance Foldable (Either a) where foldMap _ _ = error "Folding Either? Naah, I don't think this is a good idea."

This would change the semantic of every

forM_ myeither $ \i -> ....

in existing code to silent errors.

Hell no.

-Edward

On 02.03.2017 17:48, David Feuer wrote:

The problem is that we'd then lose the perfectly good Traversable instance, which would be sad.

On Mar 2, 2017 11:23 AM, "Andreas Abel" <andreas.abel@ifi.lmu.de <mailto:andreas.abel@ifi.lmu.de> <mailto:andreas.abel@ifi.lmu.de <mailto:andreas.abel@ifi.lmu.de>>> wrote:

Today a student came to me wondering why a certain function produced a regular result, where he had expected an error. Turned out he had used `concat`, but not on a lists of lists as he had thought, but on a lists of `Either a [b]`.

With the Foldable instance for Either, which considers Either a b to be a container of 0-1 elements of b, errors are happily swallowed.

I think this instance is harmful and should be deprecated (and later removed) from base.

There are similarly pointless Foldable instances as well.

See a discussion one year ago, which was heated, but had no consequences.

https://mail.haskell.org/pipermail/libraries/2016-February/026678.html <https://mail.haskell.org/pipermail/libraries/2016-February/026678.html>

<https://mail.haskell.org/pipermail/libraries/2016-February/026678.html <https://mail.haskell.org/pipermail/libraries/2016-February/026678.html>>

-- Andreas Abel <>< Du bist der geliebte Mensch.

Department of Computer Science and Engineering Chalmers and Gothenburg University, Sweden

andreas.abel@gu.se <mailto:andreas.abel@gu.se> <mailto:andreas.abel@gu.se <mailto:andreas.abel@gu.se>> http://www.cse.chalmers.se/~abela/ <http://www.cse.chalmers.se/~abela/> <http://www.cse.chalmers.se/~abela/ <http://www.cse.chalmers.se/~abela/>> _______________________________________________ Libraries mailing list Libraries@haskell.org <mailto:Libraries@haskell.org> <mailto:Libraries@haskell.org <mailto:Libraries@haskell.org>> http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries <http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries> <http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries <http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries>>

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Department of Computer Science and Engineering Chalmers and Gothenburg University, Sweden

andreas.abel@gu.se <mailto:andreas.abel@gu.se> http://www.cse.chalmers.se/~abela/ <http://www.cse.chalmers.se/~abela/> _______________________________________________ Libraries mailing list Libraries@haskell.org <mailto:Libraries@haskell.org> http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries <http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries>

-- Andreas Abel <>< Du bist der geliebte Mensch. Department of Computer Science and Engineering Chalmers and Gothenburg University, Sweden andreas.abel@gu.se http://www.cse.chalmers.se/~abela/

Sadly, doing so in Data.List would lead to massive breakage as Data.List is specified by the report and is very, very widely imported unqualified just to get at things like sort. -Edward On Thu, Mar 2, 2017 at 2:28 PM, David Feuer <david.feuer@gmail.com> wrote:

Yes. That is an excellent plan. I'd love to call it Data.List, but others will disagree.

On Mar 2, 2017 2:17 PM, "Andreas Abel" <andreas.abel@ifi.lmu.de> wrote:

...

We could have a module

Data.List.ReallyJustListsAndNotSomeThingMoreGeneric

which implements concat and friends just for lists and could be imported if one wants to have the list operations. Currently,

import qualified Data.List as List

does not give on the list operations as e.g.

List.concat

but the generic ones.

See also http://ghc.haskell.org/trac/ghc/ticket/13345

On 02.03.2017 20:02, Edward Kmett wrote:

...

On Thu, Mar 2, 2017 at 11:59 AM, Andreas Abel <andreas.abel@ifi.lmu.de <mailto:andreas.abel@ifi.lmu.de>> wrote:

Ok, Foldable is a formal condition for Traversable, but not actually used in the implementation of Traversable Either. This still leaves room to implement Foldable for Either by

instance Foldable (Either a) where foldMap _ _ = error "Folding Either? Naah, I don't think this is a good idea."

This would change the semantic of every

forM_ myeither $ \i -> ....

in existing code to silent errors.

Hell no.

-Edward

On 02.03.2017 17:48, David Feuer wrote:

The problem is that we'd then lose the perfectly good Traversable instance, which would be sad.

On Mar 2, 2017 11:23 AM, "Andreas Abel" <andreas.abel@ifi.lmu.de <mailto:andreas.abel@ifi.lmu.de> <mailto:andreas.abel@ifi.lmu.de <mailto:andreas.abel@ifi.lmu.de>>> wrote:

Today a student came to me wondering why a certain function produced a regular result, where he had expected an error. Turned out he had used `concat`, but not on a lists of lists as he had thought, but on a lists of `Either a [b]`.

With the Foldable instance for Either, which considers Either a b to be a container of 0-1 elements of b, errors are happily swallowed.

I think this instance is harmful and should be deprecated (and later removed) from base.

There are similarly pointless Foldable instances as well.

See a discussion one year ago, which was heated, but had no consequences.

https://mail.haskell.org/pipermail/libraries/2016-February/0 26678.html <https://mail.haskell.org/pipermail/libraries/2016-February/ 026678.html>

<https://mail.haskell.org/pipermail/libraries/2016-February/ 026678.html <https://mail.haskell.org/pipermail/libraries/2016-February/ 026678.html>>

-- Andreas Abel <>< Du bist der geliebte Mensch.

Department of Computer Science and Engineering Chalmers and Gothenburg University, Sweden

andreas.abel@gu.se <mailto:andreas.abel@gu.se> <mailto:andreas.abel@gu.se <mailto:andreas.abel@gu.se>> http://www.cse.chalmers.se/~abela/ <http://www.cse.chalmers.se/~abela/> <http://www.cse.chalmers.se/~abela/ <http://www.cse.chalmers.se/~abela/>> _______________________________________________ Libraries mailing list Libraries@haskell.org <mailto:Libraries@haskell.org> <mailto:Libraries@haskell.org <mailto:Libraries@haskell.org>> http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries <http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries> <http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries <http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries>>

-- Andreas Abel <>< Du bist der geliebte Mensch.

Department of Computer Science and Engineering Chalmers and Gothenburg University, Sweden

andreas.abel@gu.se <mailto:andreas.abel@gu.se> http://www.cse.chalmers.se/~abela/ <http://www.cse.chalmers.se/~a bela/> _______________________________________________ Libraries mailing list Libraries@haskell.org <mailto:Libraries@haskell.org> http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries <http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries>

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Department of Computer Science and Engineering Chalmers and Gothenburg University, Sweden

andreas.abel@gu.se http://www.cse.chalmers.se/~abela/

On Fri, 3 Mar 2017, Ivan Lazar Miljenovic wrote:

On 3 March 2017 at 06:28, David Feuer <david.feuer@gmail.com> wrote:

...

Yes. That is an excellent plan. I'd love to call it Data.List, but others will disagree.

Same here.

I wasn't that happy with the FTP proposal when it first came up due to potential confusions (primarily due to naming conventions like "length"), but now that it's done we shouldn't go back.

We should have at least compiler warnings for those who care.

On Sat, Mar 18, 2017 at 03:21:34PM -0400, Carter Schonwald wrote:

for what?

lets just have a learners / grumpy people prelude

This has been a respectful thread from both sides, why mar it with snarky remarks?

On 18/03/17 19:49, Henning Thielemann wrote:

On Sat, 18 Mar 2017, Carter Schonwald wrote:

...

for what?

A warning if someone e.g. calls 'length (a,b)', or more generally, if certain instances are used. _______________________________________________ Libraries mailing list Libraries@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries

Please no. Many of us like our code Wall-clean while still being able to write polymorphic functions. Adding more warnings that are often triggered by correct code (redundant constraints, anyone?) only leads to more headache. You could make that an hlint rule on the other hand.

I'm +1 on offering the warning and very very -1 on including it in -Wall. On Mar 18, 2017 6:19 PM, "Henning Thielemann" <lemming@henning-thielemann.de> wrote:

On Sat, 18 Mar 2017, Lana Black wrote:

On 18/03/17 19:49, Henning Thielemann wrote:

...

...

On Sat, 18 Mar 2017, Carter Schonwald wrote:

for what?

...

A warning if someone e.g. calls 'length (a,b)', or more generally, if certain instances are used.

Please no. Many of us like our code Wall-clean while still being able to write polymorphic functions.

Whether the warning is enabled by Wall is a different question. Though I think it should.

https://ghc.haskell.org/trac/ghc/ticket/11796 _______________________________________________ Libraries mailing list Libraries@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries

On Sun, 19 Mar 2017, Tony Morris wrote:

I will fork the compiler before I endure that degree of failure.

That's a reasonable approach. If you would have forked the compiler in order to implement the FTP and all the Foldable tuple instances only in the fork then we would not need the warnings, at all.

El 18 mar 2017, a las 16:01, Lana Black <lanablack@amok.cc> escribió:

...

On 18/03/17 19:49, Henning Thielemann wrote:

...

On Sat, 18 Mar 2017, Carter Schonwald wrote:

for what?

A warning if someone e.g. calls 'length (a,b)', or more generally, if certain instances are used. _______________________________________________ Libraries mailing list Libraries@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries

Please no. Many of us like our code Wall-clean while still being able to write polymorphic functions. Adding more warnings that are often triggered by correct code (redundant constraints, anyone?) only leads to more headache.

You could make that an hlint rule on the other hand.

Can it be a hlint rule? It seems quite difficult to predict that "length" will not ever be passed e.g. a 2-tuple in the general case, within hlint. I would also favor a warning, and happily have -Wall not include it (though I'd prefer inclusion). Tom

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Is there a clear way to implement this instance warning? I.e. given: f x = 2 * length x Can we guarantee at compile time that "f" will never be passed a 2-tuple? Tom

El 18 mar 2017, a las 20:04, Adam Bergmark <adam@bergmark.nl> escribió:

I'm on the fence about the instance existing. I'm +1 for a warning, and thus would be +1 on keeping the instance. +1 on making the warning opt-in and +1 keeping it out of -Wall.

...

On Sun, 19 Mar 2017 at 00:51 <amindfv@gmail.com> wrote:

...

El 18 mar 2017, a las 16:01, Lana Black <lanablack@amok.cc> escribió:

...

On 18/03/17 19:49, Henning Thielemann wrote:

...

On Sat, 18 Mar 2017, Carter Schonwald wrote:

for what?

A warning if someone e.g. calls 'length (a,b)', or more generally, if certain instances are used. _______________________________________________ Libraries mailing list Libraries@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries

Please no. Many of us like our code Wall-clean while still being able to write polymorphic functions. Adding more warnings that are often triggered by correct code (redundant constraints, anyone?) only leads to more headache.

You could make that an hlint rule on the other hand.

Can it be a hlint rule? It seems quite difficult to predict that "length" will not ever be passed e.g. a 2-tuple in the general case, within hlint.

I would also favor a warning, and happily have -Wall not include it (though I'd prefer inclusion).

Tom

...

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On Sun, 19 Mar 2017, amindfv@gmail.com wrote:

Is there a clear way to implement this instance warning? I.e. given:

f x = 2 * length x

Can we guarantee at compile time that "f" will never be passed a 2-tuple?

I gave an idea in the ticket: https://ghc.haskell.org/trac/ghc/ticket/11796 Defining f this way would be ok if x is polymorphic. The warning would only arise if f is applied to a pair.

El 19 mar 2017, a las 17:19, Tony Morris <tmorris@tmorris.net> escribió:

That you can't should be a hint that length for ((,) a) is very definitely 1.

To be frank, this seems like a non-sequitur. Many of us find e.g.: length (_, _) = 1 maximum (_, b) = b to be dangerous instances. We don't want to accidentally call them. We'd like a way to opt out or warn on use. The inability to opt out or be warned does not seem to me to be an argument that the above instances are good instances. Feel free to clarify if I haven't understood your point. Thanks! Tom

Simply, use a different function, not length, which is well-defined for ((,) a) and other instances.

...

On Mon, Mar 20, 2017 at 3:15 AM, <amindfv@gmail.com> wrote: Is there a clear way to implement this instance warning? I.e. given:

f x = 2 * length x

Can we guarantee at compile time that "f" will never be passed a 2-tuple?

Tom

...

El 18 mar 2017, a las 20:04, Adam Bergmark <adam@bergmark.nl> escribió:

I'm on the fence about the instance existing. I'm +1 for a warning, and thus would be +1 on keeping the instance. +1 on making the warning opt-in and +1 keeping it out of -Wall.

...

On Sun, 19 Mar 2017 at 00:51 <amindfv@gmail.com> wrote:

...

El 18 mar 2017, a las 16:01, Lana Black <lanablack@amok.cc> escribió:

...

On 18/03/17 19:49, Henning Thielemann wrote:

> On Sat, 18 Mar 2017, Carter Schonwald wrote: > > for what?

A warning if someone e.g. calls 'length (a,b)', or more generally, if certain instances are used. _______________________________________________ Libraries mailing list Libraries@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries

Please no. Many of us like our code Wall-clean while still being able to write polymorphic functions. Adding more warnings that are often triggered by correct code (redundant constraints, anyone?) only leads to more headache.

You could make that an hlint rule on the other hand.

Can it be a hlint rule? It seems quite difficult to predict that "length" will not ever be passed e.g. a 2-tuple in the general case, within hlint.

I would also favor a warning, and happily have -Wall not include it (though I'd prefer inclusion).

Tom

...

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Sadly, hlint remains fairly (er.. completely?) ignorant of the types of an expression. It has no idea what is in scope, and just provides syntax-directed guidance, so hlint isn't the answer here. That said, there does seem to be a plausible solution available. I have zero objection to adding {-# POISON #-} pragma that users could include in their own code that just makes it so that whatever instance is named inside of it becomes unusable. You could view it as setting up an ambiguous instance declaration that is currently impossible to write, so any attempt to use the instance would complain about ambiguous instances (or preferably about the instance being poisoned and name the source of the poison). This would make the status quo the norm, but then you could modify your custom Prelude or what have you to {-# POISON instance Foldable ((,) e) #-} and then any code that transitively depended on your Prelude would complain if it attempted to use that instance. Such a poison pragma should be sound with respect to GHC's internals. It is effectively making an extra instance just to cause conflicts later on in the instance selection process. This is the same as if we had Foldable the class being defined in one module, the real instance in another, someone's poison instance in a third, then a fourth module that imports the whole diamond. Any attempt to use the instance that is defined in two contradictory ways in that 4th module today will complain. Folks who care about these instances would then have the choice about whether to avoid any package that poisoned any instances they cared about, and folks who fee strongly about this issue would be able to live in a world where these instances didn't affect any code they wrote and if they didn't want to turn off users that care about being poisoned, could turn on the poisoning through cabal flags so that they don't infect their API but get an opt-in internal consistency check for local compilation. -Edward Kmett P.S. The term poison above derives from the C convention of poisoning names you don't want to see in your code due to safety considerations. e.g. In GCC you can do so with #pragma GCC poison printf sprintf fprintf On Sat, Mar 18, 2017 at 8:51 PM, <amindfv@gmail.com> wrote:

...

El 18 mar 2017, a las 16:01, Lana Black <lanablack@amok.cc> escribió:

...

On 18/03/17 19:49, Henning Thielemann wrote:

...

On Sat, 18 Mar 2017, Carter Schonwald wrote:

for what?

A warning if someone e.g. calls 'length (a,b)', or more generally, if certain instances are used. _______________________________________________ Libraries mailing list Libraries@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries

Please no. Many of us like our code Wall-clean while still being able to write polymorphic functions. Adding more warnings that are often triggered by correct code (redundant constraints, anyone?) only leads to more headache.

You could make that an hlint rule on the other hand.

Can it be a hlint rule? It seems quite difficult to predict that "length" will not ever be passed e.g. a 2-tuple in the general case, within hlint.

I would also favor a warning, and happily have -Wall not include it (though I'd prefer inclusion).

Tom

...

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Ed, If i understand you correctly then this poisoning doesn't scale. If I want to use it in my library I'm also forcing all my users to avoid these functions and it would split hackage in half. Effectively I'd only be able to use it for private code and applications. Or am I misunderstanding? You can already do this in poisoning in a solid way by defining e.g. `class Unsatisfiable; instance Unsatisfiable => Show (a -> b) where show = undefined` and not exporting Unsatisfiable. Cheers, Adam On Mon, 20 Mar 2017 at 01:56 <amindfv@gmail.com> wrote:

Edward: this is clever! I can't at the moment see any reason this wouldn't work for all parties: those who want the instances can have them, those who don't can "turn them off," and beginners can start with a prelude that turns them off also.

We will need to be clear about which packages (transitively) poison instances.

Tom

El 19 mar 2017, a las 19:21, Edward Kmett <ekmett@gmail.com> escribió:

Sadly, hlint remains fairly (er.. completely?) ignorant of the types of an expression. It has no idea what is in scope, and just provides syntax-directed guidance, so hlint isn't the answer here.

That said, there does seem to be a plausible solution available.

I have zero objection to adding {-# POISON #-} pragma that users could include in their own code that just makes it so that whatever instance is named inside of it becomes unusable. You could view it as setting up an ambiguous instance declaration that is currently impossible to write, so any attempt to use the instance would complain about ambiguous instances (or preferably about the instance being poisoned and name the source of the poison).

This would make the status quo the norm, but then you could modify your custom Prelude or what have you to {-# POISON instance Foldable ((,) e) #-} and then any code that transitively depended on your Prelude would complain if it attempted to use that instance.

Such a poison pragma should be sound with respect to GHC's internals. It is effectively making an extra instance just to cause conflicts later on in the instance selection process. This is the same as if we had Foldable the class being defined in one module, the real instance in another, someone's poison instance in a third, then a fourth module that imports the whole diamond. Any attempt to use the instance that is defined in two contradictory ways in that 4th module today will complain.

Folks who care about these instances would then have the choice about whether to avoid any package that poisoned any instances they cared about, and folks who fee strongly about this issue would be able to live in a world where these instances didn't affect any code they wrote and if they didn't want to turn off users that care about being poisoned, could turn on the poisoning through cabal flags so that they don't infect their API but get an opt-in internal consistency check for local compilation.

-Edward Kmett

P.S. The term poison above derives from the C convention of poisoning names you don't want to see in your code due to safety considerations. e.g. In GCC you can do so with

#pragma GCC poison printf sprintf fprintf

On Sat, Mar 18, 2017 at 8:51 PM, <amindfv@gmail.com> wrote:

...

El 18 mar 2017, a las 16:01, Lana Black <lanablack@amok.cc> escribió:

...

On 18/03/17 19:49, Henning Thielemann wrote:

...

On Sat, 18 Mar 2017, Carter Schonwald wrote:

for what?

A warning if someone e.g. calls 'length (a,b)', or more generally, if certain instances are used. _______________________________________________ Libraries mailing list Libraries@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries

Please no. Many of us like our code Wall-clean while still being able to write polymorphic functions. Adding more warnings that are often triggered by correct code (redundant constraints, anyone?) only leads to more headache.

You could make that an hlint rule on the other hand.

Can it be a hlint rule? It seems quite difficult to predict that "length" will not ever be passed e.g. a 2-tuple in the general case, within hlint.

I would also favor a warning, and happily have -Wall not include it (though I'd prefer inclusion).

Tom

...

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On Sun, Mar 19, 2017 at 9:33 PM, Adam Bergmark <adam@bergmark.nl> wrote:

Ed, If i understand you correctly then this poisoning doesn't scale. If I want to use it in my library I'm also forcing all my users to avoid these functions and it would split hackage in half. Effectively I'd only be able to use it for private code and applications. Or am I misunderstanding?

This is why I mentioned at the end that if you're doing this for a library you probably only want to enable poisoning with a cabal build flag. That way you can use it to verify that you build clean even with the poison enabled without crippling your users. Think of it like -Werror. You don't ship cabal packages with it enabled but you often use it locally. For classroom teaching scenarios using a custom prelude that poisons things fits the bill. If you are concerned about certain instances leaking into your private code, you can also poison willy-nilly, and nobody needs care. If you poison behind a flag it doesn't leak out into the ecosystem, and if you choose not to poison behind a flag, then people can choose to accept or reject your poisoned instances on their own merits. e.g. async had an instance of Monad for Concurrently that was incompatible with its Applicative until recently. Fortunately, Simon Marlow was willing to listen to reason. However, had he not been, it is a situation where I could bring myself to resort to poison. Er... that sounds rather more gruesome than intended. Sorry, Simon. :) You can already do this in poisoning in a solid way by defining e.g. `class

Unsatisfiable; instance Unsatisfiable => Show (a -> b) where show = undefined` and not exporting Unsatisfiable.

Yes, if you want to ensure that an instance is never defined, but that doesn't work for things where there is already an instance in scope. You can't retroactively poison Foldable ((,) e) with this technique, as the class and instance are defined together in the same module (or the data type and instance are), as they have to be to avoid orphans, and so you never get a chance to interject a conflicting poisonous instance without GHC telling you that your instance collides with the existing instance as an error. -Edward Cheers,

Adam

On Mon, 20 Mar 2017 at 01:56 <amindfv@gmail.com> wrote:

...

Edward: this is clever! I can't at the moment see any reason this wouldn't work for all parties: those who want the instances can have them, those who don't can "turn them off," and beginners can start with a prelude that turns them off also.

We will need to be clear about which packages (transitively) poison instances.

Tom

El 19 mar 2017, a las 19:21, Edward Kmett <ekmett@gmail.com> escribió:

Sadly, hlint remains fairly (er.. completely?) ignorant of the types of an expression. It has no idea what is in scope, and just provides syntax-directed guidance, so hlint isn't the answer here.

That said, there does seem to be a plausible solution available.

I have zero objection to adding {-# POISON #-} pragma that users could include in their own code that just makes it so that whatever instance is named inside of it becomes unusable. You could view it as setting up an ambiguous instance declaration that is currently impossible to write, so any attempt to use the instance would complain about ambiguous instances (or preferably about the instance being poisoned and name the source of the poison).

This would make the status quo the norm, but then you could modify your custom Prelude or what have you to {-# POISON instance Foldable ((,) e) #-} and then any code that transitively depended on your Prelude would complain if it attempted to use that instance.

Such a poison pragma should be sound with respect to GHC's internals. It is effectively making an extra instance just to cause conflicts later on in the instance selection process. This is the same as if we had Foldable the class being defined in one module, the real instance in another, someone's poison instance in a third, then a fourth module that imports the whole diamond. Any attempt to use the instance that is defined in two contradictory ways in that 4th module today will complain.

Folks who care about these instances would then have the choice about whether to avoid any package that poisoned any instances they cared about, and folks who fee strongly about this issue would be able to live in a world where these instances didn't affect any code they wrote and if they didn't want to turn off users that care about being poisoned, could turn on the poisoning through cabal flags so that they don't infect their API but get an opt-in internal consistency check for local compilation.

-Edward Kmett

P.S. The term poison above derives from the C convention of poisoning names you don't want to see in your code due to safety considerations. e.g. In GCC you can do so with

#pragma GCC poison printf sprintf fprintf

On Sat, Mar 18, 2017 at 8:51 PM, <amindfv@gmail.com> wrote:

...

El 18 mar 2017, a las 16:01, Lana Black <lanablack@amok.cc> escribió:

...

On 18/03/17 19:49, Henning Thielemann wrote:

...

On Sat, 18 Mar 2017, Carter Schonwald wrote:

for what?

A warning if someone e.g. calls 'length (a,b)', or more generally, if certain instances are used. _______________________________________________ Libraries mailing list Libraries@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries

Please no. Many of us like our code Wall-clean while still being able to write polymorphic functions. Adding more warnings that are often triggered by correct code (redundant constraints, anyone?) only leads to more headache.

You could make that an hlint rule on the other hand.

Can it be a hlint rule? It seems quite difficult to predict that "length" will not ever be passed e.g. a 2-tuple in the general case, within hlint.

I would also favor a warning, and happily have -Wall not include it (though I'd prefer inclusion).

Tom

...

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The nice thing about such a module is anybody can package it up. It needs nothing special from base. During the discussion about Foldable, the option to add a Data.List.Explicit or something that contained just the monomorphic variants was brought up as something we could add, but nobody really got enthusiastic over the idea. The state of Data.List today is a messy intermediate state: https://ghc.haskell.org/trac/ghc/wiki/Prelude710/FTP#list When and if we get a resolution to https://ghc.haskell.org/trac/ghc/ticket/4879 we're likely to proceed with option 1 from that FTP page, which will at least avoid Data.List.concat having the too general type, by dint of it no longer existing. -Edward On Thu, Mar 2, 2017 at 2:17 PM, Andreas Abel <andreas.abel@ifi.lmu.de> wrote:

We could have a module

Data.List.ReallyJustListsAndNotSomeThingMoreGeneric

which implements concat and friends just for lists and could be imported if one wants to have the list operations. Currently,

import qualified Data.List as List

does not give on the list operations as e.g.

List.concat

but the generic ones.

See also http://ghc.haskell.org/trac/ghc/ticket/13345

On 02.03.2017 20:02, Edward Kmett wrote:

...

On Thu, Mar 2, 2017 at 11:59 AM, Andreas Abel <andreas.abel@ifi.lmu.de <mailto:andreas.abel@ifi.lmu.de>> wrote:

Ok, Foldable is a formal condition for Traversable, but not actually used in the implementation of Traversable Either. This still leaves room to implement Foldable for Either by

instance Foldable (Either a) where foldMap _ _ = error "Folding Either? Naah, I don't think this is a good idea."

This would change the semantic of every

forM_ myeither $ \i -> ....

in existing code to silent errors.

Hell no.

-Edward

On 02.03.2017 17:48, David Feuer wrote:

The problem is that we'd then lose the perfectly good Traversable instance, which would be sad.

On Mar 2, 2017 11:23 AM, "Andreas Abel" <andreas.abel@ifi.lmu.de <mailto:andreas.abel@ifi.lmu.de> <mailto:andreas.abel@ifi.lmu.de

<mailto:andreas.abel@ifi.lmu.de>>> wrote:

Today a student came to me wondering why a certain function produced a regular result, where he had expected an error. Turned out he had used `concat`, but not on a lists of lists as he had thought, but on a lists of `Either a [b]`.

With the Foldable instance for Either, which considers Either a b to be a container of 0-1 elements of b, errors are happily swallowed.

I think this instance is harmful and should be deprecated (and later removed) from base.

There are similarly pointless Foldable instances as well.

See a discussion one year ago, which was heated, but had no consequences.

https://mail.haskell.org/pipermail/libraries/2016-February/ 026678.html <https://mail.haskell.org/pipermail/libraries/2016-February/ 026678.html>

<https://mail.haskell.org/pipermail/libraries/2016-February/ 026678.html <https://mail.haskell.org/pipermail/libraries/2016-February/ 026678.html>>

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andreas.abel@gu.se http://www.cse.chalmers.se/~abela/

I'd also support this.

El 3 mar 2017, a las 11:23, David Feuer <david.feuer@gmail.com> escribió:

Yes. I completely agree that the goal of being able to import Data.Foldable unqualified was the wrong goal from the start. That goal is sensible for Data.Traversable, I think, since Traversable has very few methods and since instances are sufficiently constrained to prevent *most* of these problems. But it falls apart for Data.Foldable, which is too "wild". I for one would be quite happy to banish the lion's share of Foldable operations back to Data.Foldable, so the Prelude would re-export only Data.Foldable (Foldable (foldMap)), and Data.List would export only list-specific functions.

On Fri, Mar 3, 2017 at 10:32 AM, lennart spitzner <lsp@informatik.uni-kiel.de> wrote:

...

(This is a reply to this topic in general, not specific to Andreas' suggestion. Also, this argument should generalize well to other Foldable methods/instances.)

I don't like the behaviour you observe either. Still the analysis/proposal ignores half of the cause - this has been (at least indirectly) pointed out repeatedly in this and past threads already, but I'd like to state it clearly because people also repeatedly seem to ignore the fact.

In order for the expression `concat (xs :: [Either a [b]])` to be accepted, two things need to be the case: Firstly the concat in scope needs to be one that is expressed in terms of Foldable. Secondly there needs to be a Foldable instance for Either. (Technically it needs to be in scope too, but the difference matters only if the instance was an orphan, but I'll assume for now that "orphanization" is not the solution.) The first reason is of course connected to the design of the default Prelude - which I suppose was responsible for getting the specific `concat` into scope here.

We could change either of these causes: Change what is exported from Prelude or Remove the instance (and both are breaking compatibility). As a consequence, I find going from "uses of concat(/length/maximum/..) may be confusing/cause bugs" to "we need to remove the instance" to be a weak conclusion.

Further, if don't overlook something, removing an instance (or removing a particular method from an instance) always would create at least as much trouble as changing what is exported from the Prelude would, as the types of concat/length/maximum/.. would necessarily change as a consequence of the former. So to all those arguing in favour of removing methods/instances: What does removing/prohibiting instances give in addition to a change to the type of concat(/length/..) as included unqualified in the default Prelude? In this direction, I have not seen any convincing arguments. (And sorry, I have not read all messages in the previous discussions - it was a bit too much to read for too little general enlightenment.)

(My solution is to use custom Preludes for personal projects with exclusively qualified imports, e.g. List.length :: [a] -> Int and Foldable.length :: Foldable t => t a -> Int. Works well, but I see the issues that a custom Prelude is yet another hurdle for beginners.)

-- lennart

...

On 02/03/17 17:22, Andreas Abel wrote: Today a student came to me wondering why a certain function produced a regular result, where he had expected an error. Turned out he had used `concat`, but not on a lists of lists as he had thought, but on a lists of `Either a [b]`.

With the Foldable instance for Either, which considers Either a b to be a container of 0-1 elements of b, errors are happily swallowed.

I think this instance is harmful and should be deprecated (and later removed) from base.

There are similarly pointless Foldable instances as well.

See a discussion one year ago, which was heated, but had no consequences.

https://mail.haskell.org/pipermail/libraries/2016-February/026678.html

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I think one of the reasons this debate continues to crop up is that there is a fundamental tension in the design of Haskell: Haskell strives to be a) richly typed, leading to "if it compiles, it works!" b) as general as possible, leading to wide applicability of polymorphic functions These two laudable goals work against each other. In many instances, we will have to choose between them, and different people will have different judgment calls. Perhaps it would bring some of these issues into starker relief if we consider the following instance:

instance Num Bool where (+) = (||) a - b = not (a == b) (*) = (&&)

abs = id signum = id

negate = not

fromInteger 0 = False fromInteger _ = True

This instance is not wholly specious: it's based on well-studied Boolean algebra (at least, the definitions for (+) and (*) are), and it obeys the one documented law for Num instances. It's not fully canonical in the way that, say, the `Foldable ((,) a)` instance is, but not all instances in base are fully canonical, anyway. I venture to say it would find use in practice. But, at the same time, I imagine most Haskellers would find this instance distasteful, as it is too much in service of goal (b) and strays too far from goal (a). About the current debate: my own tendency is to lean toward (a), but the community (now that we have FTP) appears to be drifting more toward (b). Though I see its merits -- and I take advantage of it in my own code -- I've never liked FTP and probably never will. That said, changing Either's Foldable instance to use `error` seems like unnecessary code breakage and goes against the apparent design principles at work in `base`. But, to me, the most important aspect of a debate such as this one is that there is no one right answer, just differing viewpoints, scattered along a spectrum from (a) to (b) (surely among many other axes). We all must live with one `base`, so we all have skin in the game and the debate may be worth continuing. I just hesitate to call any comment I've seen here "wrong". Richard

On Mar 3, 2017, at 12:45 PM, Kris Nuttycombe <kris.nuttycombe@gmail.com> wrote:

This is an interesting assertion; I don't believe that these goals work against one another at all. In fact, I find that (b) leads to "if it compiles, it works" much more often than not. John De Goes goes into depth on this principle here: http://degoes.net/articles/insufficiently-polymorphic <http://degoes.net/articles/insufficiently-polymorphic> where he points out "Monomorphic code is much more likely to be incorrect than polymorphic code, because for every type signature, there are many more possible implementations.".

Ah -- very good point. This is true for the *implementation* of a polymorphic function, where a polymorphic type signature beautifully restricts what the function can do. But it is not true for monomorphic *uses* of a polymorphic function, where the generality can lead to an unexpected instance selection and thus runtime behavior. Richard

But it is not true for ... *uses* of a polymorphic function

Which is confirmed by the present casus. The new highly polymorphic concat is a versatile weapon to shoot yourself in the foot. (However, the weapon designer can sleep even more soundly now in the confidence that his product does not malfunction.) Anyway, for what it's worth, I retreat my proposal (or "proposal"). Sh*t has happened. A monomorphic List library would make sense, though. On 03.03.2017 18:48, Richard Eisenberg wrote:

...

On Mar 3, 2017, at 12:45 PM, Kris Nuttycombe <kris.nuttycombe@gmail.com <mailto:kris.nuttycombe@gmail.com>> wrote:

This is an interesting assertion; I don't believe that these goals work against one another at all. In fact, I find that (b) leads to "if it compiles, it works" much more often than not. John De Goes goes into depth on this principle here: http://degoes.net/articles/insufficiently-polymorphic where he points out "Monomorphic code is much more likely to be incorrect than polymorphic code, because for every type signature, there are many more possible implementations.".

Ah -- very good point. This is true for the *implementation* of a polymorphic function, where a polymorphic type signature beautifully restricts what the function can do. But it is not true for monomorphic *uses* of a polymorphic function, where the generality can lead to an unexpected instance selection and thus runtime behavior.

Richard

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I am opposed too. Aside from the technical reasons, which have already been discussed and decided, confused students dictating language and library design has never worked out well. On Thu, Mar 9, 2017 at 12:49 PM, Andrew Martin <andrew.thaddeus@gmail.com> wrote:

I am opposed to making this change. I have the same objections that others on this thread do. It's unfortunate that the concat put in scope by the Prelude is the one from Data.Foldable and not the one from GHC.OldList. I do not think that removing Foldable instances is an appropriate solution to this problem though.

On Thu, Mar 2, 2017 at 11:22 AM, Andreas Abel <andreas.abel@ifi.lmu.de> wrote:

...

Today a student came to me wondering why a certain function produced a regular result, where he had expected an error. Turned out he had used `concat`, but not on a lists of lists as he had thought, but on a lists of `Either a [b]`.

With the Foldable instance for Either, which considers Either a b to be a container of 0-1 elements of b, errors are happily swallowed.

I think this instance is harmful and should be deprecated (and later removed) from base.

There are similarly pointless Foldable instances as well.

See a discussion one year ago, which was heated, but had no consequences.

https://mail.haskell.org/pipermail/libraries/2016-February/026678.html

-- Andreas Abel <>< Du bist der geliebte Mensch.

Department of Computer Science and Engineering Chalmers and Gothenburg University, Sweden

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