Author here: commas are necessary to separate list elements in any language where function application uses whitespace (i.e. Haskell-style function application). If you're not familiar with the syntax, an expression like `f x y z` is a function `f` applied to three arguments (`x`, `y`, and `z`), analogous to `f(x, y, z)` in a more traditional language.

Nix made this mistake of using whitespace to separate list elements AND using whitespace to separate a function from its argument and it is a very common pain point for new users, because they will write something like this:

    [ theFunction theFunction'sArgument ]

... thinking it will be parsed as:

    [ (theFunction theFunction'sArgument) ]

... but it actually gets parsed as a list with two elements, leading to a bizarre type error.