Permitting trailing commas for record syntax ADT declarations

[Herbert Valerio Riedel]

On 2014-09-25 at 11:18:01 +0200, Simon Peyton Jones wrote:

> That would suggest continuing to make extra commas illegal in literal
> lists, for now anyway.  That’s a conservative choice, which is usually
> the side to err on when it’s a toss-up.

I'd just like to point out, that lists are something that you may want
to tweak regularily in code (while tuple are rather fixed-size entities whose
size is extended less frequently)

Consider the following JSON example:

foo = object [
   "key1" .= True,
   "key2" .= 123,
   "key3" .= (),
   ]

This is code where I often tend to add/remove lines, and ExtraCommas
would greatly reduce the diff-noise, and which I really miss from
languages such as Python which allow me to use trailing commas.