I wonder if people's aversion to commas first has something to do with the fact that commas have a very familiar use in ordinary writing, and there they always follow a word. It is very common to see BNF grammars written with leading vertical bars, and no one seems to mind that -- in fact it would be unusual to use trailing vertical bars.

By the way, a bunch of languages permit a trailing comma after the last element of a list. This is also nice because it means that if you put each list item on a line, the list items all have the same format (thing plus comma). With the Haskell syntax, you lose this, but there is no reason a language couldn't support a leading comma before the first element of a list.

Not if the language allows trailing commas on the last line, like Python and Go do.

Then you have a comma at the end in ALL lines, including the last one, without an issue. That was what I proposed above (and lots of languages do it, I think JS is adding it too).

   - { old-one
   + { new-one
   + , old-one

This doesn't break anything, but it makes looking at "git blame" so much easier.