instance {Semigroup, Monoid} (Bag a) ?

[Richard Eisenberg]

Hi devs,

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.

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 favor adding them, and slipped them into !5509 (a big commit with lots of other stuff). Alfredo rightly wondered whether this decision deserved more scrutiny, and so I'm asking the question here.

What do we think?

Thanks,
Richard