docs: def raw(cls, exp, files=None):
          """
          :param str|unicode exp: Haskell expression to evaluate.
          :param dict[str|unicode, str|unicode] files: Dictionary of file names->contents
          :rtype: dict
          """
          payload = {'exp': exp}
          if files:
              # TODO: Implement stdin.
              stdin = []
              payload['args'] = json.dumps([stdin, files])
          try:
              return requests.get('http://tryhaskell.org/eval', params=payload).json()
          except ValueError as e:
              raise cls.Error(e)