refmt: sometimes new lines are unnecessary in switch cases

I'm just starting with Reason and I'm using Reason Language Server with coc.nvim. I'm playing with the formatting feature using a simple function from Reason docs, though I'm not finding it pretty for this function in particular:

let isVowel    = (theChar) =>
    switch (theChar) {
    | 'a' | 'e' | 'i' | 'o' | 'u' | 'y' => true
    | _ => false
    };

Formatting that results in:

let isVowel = theChar =>
  switch (theChar) {
  | 'a'
  | 'e'
  | 'i'
  | 'o'
  | 'u'
  | 'y' => true
  | _ => false
  };

I really didn't wish the vowels options to split in individual lines for this case. I was basically expecting to just fix the whitespace in let isVowel = (theChar). So I tried range formatting just in that part but the server doesn't format it alone, it requires the whole function for formatting it.

I'm not sure though how this can be tackled since splitting switch cases in multiple lines may be useful in most occasions.

Issue migrated from https://github.com/jaredly/reason-language-server/issues/211.

Start with refmt and reproduce the Reason example from the issue, comparing the current output with the requested formatting. Trace the switch-case formatting behavior and determine a rule that avoids unnecessary line breaks without removing useful multiline formatting; done means the example formats as expected and existing switch-case behavior remains intact.