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@haskell.org
http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs
