This is a by-product of Dhall coming from the Haskell community where this formatting was preferred instead of final trailing commas. It has been internalised and was carried on. I say this as someone who looks at this code and thinks "This is totally fine." -- but I admit, I'm environmentally damaged.

Yes. And, alas, you can't just allow a final trailing comma everywhere in Haskell.

Eg ("Foo", 2,) is different from ("Foo", 2) in Haskell thanks to TupleSections. For innocent bystanders: in Haskell ("Foo", 2) is the tuple you'd expect it to be. But ("Foo", 2,) is a function that takes another argument and creates a three-tuple. A Python equivalent would be

lambda x: ("Foo", 2, x)

Or allow a leading comma:

    {
    , home       = "/home/bill"
    , privateKey = "/home/bill/.ssh/id_ed25519"
    , publicKey  = "/home/blil/.ssh/id_ed25519.pub"
    }

Or, to make it markdown-ish:

    {
    - home       = "/home/bill"
    - privateKey = "/home/bill/.ssh/id_ed25519"
    - publicKey  = "/home/blil/.ssh/id_ed25519.pub"
    }

This is just to play with the idea that leading punctuation may be preferable because it all aligns in the same column.

> Or allow a leading comma

This actually works just fine.

I would actually prefer no comma's

  {:home        "/home/bill"
   :private-key "/home/bill/.ssh/id_ed25519"
   :public-key  "/home/bill/.ssh/id_ed25519.pub"}

That's the same as a leading comma.

Are you mixing up the colons with the separator?

This style is common in Haskell, Nix and Dhall. It also sees more "mainstream" use, e.g. in SQL.

I much prefer this style, I use it wherever I can because I can comment out lines without a trailing comma causing a syntax error. SQL, Ruby… wherever I can.

Try it, I doubt you'll go back (unless someone's stupid parser doesn't let you).

You can do the same, if your language allows a final comma.

(And that's what the comment you reply to suggests: make Dhall allow a final comma.)

What's the point of allowing a trailing comma? A delimiter symbol is a delimiter symbol, be it a an opening curly brace or a comma between elements, and one can simply align delimeters.

The main point is to make git diffs smaller when adding or removing elements, I think.

I feel this is the superior way to format something like that. Trailing commas are nothing but an ugly hack.

Yeah, I also really dislike this mixing up of indentation levels because of a language limitation. The "{" denotes a container, and the "," a separation between items of said container, for me it makes sense for them to be nested a level deeper

That's one way to interpret things, but not the only way.

In practice, this style reads just fine once you get used to it. Indentation is mostly there as a human convenience, so just has to work well with human brains (and be understood by a computer, if it's significant), but doesn't have to necessarily follow some abstract unified theory of syntax trees.

both symbols are delimiters, one can choose to align them together even when they are not the same character. Opening and closing braces aren't the same character either, but people have been aligning them for ages, I don't see a reason why commas, while being part of the same expression, should not follow the same principle.

For me it's not about being the same character (like you've mentioned, opening and closing braces aren't either), it's about commas and braces indicating different things in the hierarchy. Not to mention the symmetry breaking: an opening brace together with data in a line, then a lonely one at the bottom.

In a series of declarations, the lonely closing brace at the bottom can be treated as a substitute for an empty line between two entries, as it produces a similar sparse spacing.

Trailing comma is ugly as sin - I love the little "house" my leading commas put my record into :)