Map mashing function

[Henning Thielemann]

On Mon, 8 Aug 2016, David Feuer wrote:

> I was wrong. It's possible to write the general safe merge function in 
> Haskell 98, by replacing the GADT with a record of functions. Sorry for 
> the confusion!

I would prefer that very much!

The discussion about it seemed to miss the libraries mailing list:
    https://mail.haskell.org/libraries/2016-August/027243.html