Leading is necessary to minimize diffs when you don't support trailing commas: Prelude> [1,] <interactive>:4:4: error: parse error on input ‘]’ Pr

Leading is necessary to minimize diffs when you don't support trailing commas: Prelude> [1,] <interactive>:4:4: error: parse error on input ‘]’ Prelude> [1] [1] This means if you go multiline in the "usual" style: [ 1, 2, 3 ] any time you need to append a new item, you need to modify the existing last line as well: [ 1, 2, - 3 + 3, + 4 ] Meanwhile: [ 1 , 2 , 3 +, 4 ] If you support trailing commas, you can just use one on the last line for the same effect: [ 1, 2, 3, + 4, ] Incidentally, some crazy people put the closing symbol on the last line: [ 1 , 2 , 3 ] That completely defeats the point: [ 1 , 2 -, 3 ] +, 3 +, 4 ]