instance {Semigroup, Monoid} (Bag a) ?

[chessai]

+1 to add from me, seems sensible

On Wed, Apr 14, 2021, 14:31 Viktor Dukhovni <ietf-dane at dukhovni.org> wrote:

> On Wed, Apr 14, 2021 at 06:26:38PM +0000, Richard Eisenberg wrote:
>
> > In the work on simplifying the error-message infrastructure (heavy
> > lifting by Alfredo, in cc), I've been tempted (twice!) to add
> >
> > > instance Semigroup (Bag a) where
> > >   (<>) = unionBags
> > >
> > > instance Monoid (Bag a) where
> > >   mempty = emptyBag
> >
> > to GHC.Data.Bag.
>
> I agree that the new Monoid is appropriate.
>
> > The downside to writing these is that users might be tempted to write
> > e.g. mempty instead of emptyBag, while the latter gives more
> > information to readers and induces less manual type inference (to a
> > human reader). The upside is that it means Bags work well with
> > Monoid-oriented functions, like foldMap.
>
> I don't see the possibility of writing `mempty` as an issue.  I find
> myself not infrequently writing `mempty` for, e.g., empty ByteStrings,
> rather than ByteString.empty, because while there are lots of
> type-specific "empties", they often need to be used qualified, while the
> polymorphic `mempty` is both clear and flexible.
>
> If anything, what's atypical here is that "emptyBag" has the type in its
> name.  With many other types we have:
>
>     empty :: ByteString
>     empty :: ByteString.Builder
>     empty :: Map k v
>     empty :: Set a
>     empty :: Seq a
>     empty :: Text
>     empty :: Vector a
>     ...
>
> when the type is a Monoid, it is much simpler to just use mempty.
>
> --
>     Viktor.
> _______________________________________________
> ghc-devs mailing list
> ghc-devs at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.haskell.org/pipermail/ghc-devs/attachments/20210414/bc515526/attachment.html>