instance {Semigroup, Monoid} (Bag a) ?

Viktor Dukhovni ietf-dane at
Wed Apr 14 19:29:44 UTC 2021

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.


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