msg:
{ actual, expected, asString ? false, asJSON ? false, removeDunders ? false
, safeToPrint ? true }:
let
  preProcess = v:
    if removeDunders then
      undunder v
    else if asString then
      toString v
    else if asJSON then
      builtins.toJSON v
    else
      v;

  a = preProcess actual;
  e = preProcess expected;

  undunder = v: if builtins.isAttrs v then builtins.removeAttrs v [ "__toString" "__functor" ] else v;

  out = (if safeToPrint then builtins.toJSON (undunder actual) else ''{"msg": "refusing to print"}'');

in if (a == e) then ''
  echo 'it ${msg}'
'' else
  builtins.trace "FAILED ${msg}" ''
    echo FAILED ${msg} 
    echo '${out}' | jq '.'
  ''