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