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It looks that Dhall has been inspired of the Elm language [0] and it's formatter.

It's a common practice in the Haskell community. Knowing where the creator of dhall comes from I would say that that's the source of inspiration

Author here: the syntax is inspired by all three of Haskell/PureScript/Elm

I get that this makes diffs a tiny bit nicer when adding new lines, if you don't use trailing commas, but christ it's ugly!

> commas at the start of lines

are not a requirement.

OTOH I would guess it doesn't allow a trailing comma (same problem as JSON...) so you end up with weird ugly formatting conventions

No, you can use trailing commas if you want.

Final trailing comma is not in the standard yet, but will be soon