[merijnv]

Having prefixed commas is a rather common style in the Haskell community, because it ends up nicely matching open/closing brackets/braces and lining things up.

Since the author of Dhall comes from the Haskell community, he's kept this style.

[Gabriel439]

Author here: This is correct. I'm just borrowing a Haskell convention. Also, I like this convention because it leads to vertical alignment of commas.