Infix style with large expressions about smlfmt HOT 1 OPEN

Revisiting this now that smlfmt is quite a bit more mature.

Re: don't move independent code? I think in practice this is really really difficult to accomplish without making huge sacrifices to average code readability.

As for our current handling of infix expressions, I think it's generally pretty good, but could use some improvements. Here's the current output for the above example:

val _ =
  f x
  +
  case g x of
    nil => 0
  | x :: xs => x + List.length xs

Here's also a real-world example from the smlfmt source, showing a fairly complex expression with multiple occurrences of an infix operator <&>:

This generally seems pretty good, but I think it could be improved. For example, here's an instance I find tricky to parse because it's easy to miss the <&> at #delims ... buried in the middle:

I would prefer for it to be formatted like this.

val checker =
  at #exp equal_exp <&> at #handlee equal_tok
  <&>
  at #elems (Seq.equal
    (at #pat equal_pat <&> at #arrow equal_tok
     <&> at #exp equal_exp))
  <&> at #delims (Seq.equal equal_tok)  (* on its own line *)
  <&> at #optbar (equal_op equal_tok)

This seems doable. One idea to try: ensure that, as soon as an element of an infix-chain requires a new-line to align, then all later <op> <exp> ... elements of the chain will be put on their own new-line as well.

from smlfmt.