Yes, when the commas are at the beginning of the line they are really obvious. I have used commas-first style in JavaScript, and it definitely reduced the amount of times I forgot a comma.
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.
My aversion has to do with the fact that the first element doesn't have one. The irregularity is annoying not just aesthetically, but also if you want to swap lines around, etc.
But there's always a line without a comma -- comma-last has it on the last line instead of the first. So you can have the same line-swapping problem in either arrangement. Comma first just makes it easier to spot.
Not if you allow dangling commas (trailing commas as the sibling comment calls them, I think it's the more generic term)...
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).
It's another thing to learn, and the total learning curve is the product of all of these idiosyncrasies (and features) (not the sum). And there's absolutely no advantage to comma-first.
This is only true if you insert at the head. And it's not really leading comma placement but the choice to put the first element in the line with the brace that is the culprit, though both are typical of comma-first.
With separating comma, it either affects the first or the last position, depending on where you put the commas. But those are the most common places you modify lists.
