Adding imports adds an extra comma

[cdsmith]

It is legal in Haskell to write an import list like this:

import Foo
  ( a,
    b,
    c,
  )

Note the trailing comma. In particular, Ormolu formats files this way. However, if I have an import list formatted this way, and ask HLS to add an import (for example, from the quick-actions list, or otherwise auto-importing something), I get this

import Foo
  ( a,
    b,
    c,, newImport
  )

Note the double comma at the end. Of course, this is a syntax error.

[michaelpj]

I bet this is related to https://github.com/haskell/haskell-language-server/issues/2399, in that something is not using exactprint. If we're using exactprint I wouldn't expect this kind of bug.

[kokobd]

I'm going to take a look at this.