msg:
{ actual, expected ? { }, asString ? false, asJSON ? false
, removeDunders ? false, safeToPrint ? true, throws ? false }:
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 (if throws then
      (builtins.tryEval actual).value
    else {
      inherit actual expected;
    }))
  else
    ''{"msg": "cannot be stringified ):"}'');

  success =
    if throws then (builtins.tryEval actual).success == false else (a == e);

in if success then ''
  echo 'it ${msg}'
'' else
  builtins.trace "FAILED ${msg}" (let file = builtins.toFile "value" out;
  in ''
    echo FAILED ${msg} 
    echo '${file}'
    cat '${file}' | jq '.'
  '')