docs: def _cast(self, out, request, response, peek=None):
          """ Try to convert the parameter into something WSGI compatible and set
          correct HTTP headers when possible.
          Support: False, str, unicode, dict, HTTPResponse, HTTPError, file-like,
          iterable of strings and iterable of unicodes
          """
          # Filtered types (recursive, because they may return anything)
          for testtype, filterfunc in self.castfilter:
              if isinstance(out, testtype):
                  return self._cast(filterfunc(out), request, response)
  
          # Empty output is done here
          if not out:
              response.headers['Content-Length'] = 0
              return []
          # Join lists of byte or unicode strings. Mixed lists are NOT supported
          if isinstance(out, list) and isinstance(out[0], (StringType, unicode)):
              out = out[0][0:0].join(out) # b'abc'[0:0] -> b''
          # Encode unicode strings
          if isinstance(out, unicode):
              out = out.encode(response.charset)
          # Byte Strings are just returned
          if isinstance(out, StringType):
              response.headers['Content-Length'] = str(len(out))
              return [out]
          # HTTPError or HTTPException (recursive, because they may wrap anything)
          if isinstance(out, HTTPError):
              out.apply(response)
              return self._cast(self.error_handler.get(out.status, repr)(out), request, response)
          if isinstance(out, HTTPResponse):
              out.apply(response)
              return self._cast(out.output, request, response)
  
          # Cast Files into iterables
          if hasattr(out, 'read') and 'wsgi.file_wrapper' in request.environ:
              out = request.environ.get('wsgi.file_wrapper',
              lambda x, y: iter(lambda: x.read(y), ''))(out, 1024*64)
  
          # Handle Iterables. We peek into them to detect their inner type.
          try:
              out = iter(out)
              first = out.next()
              while not first:
                  first = out.next()
          except StopIteration:
              return self._cast('', request, response)
          except HTTPResponse, e:
              first = e
          except Exception, e:
              first = HTTPError(500, 'Unhandled exception', e, format_exc(10))
              if isinstance(e, (KeyboardInterrupt, SystemExit, MemoryError))\
              or not self.catchall:
                  raise
          # These are the inner types allowed in iterator or generator objects.
          if isinstance(first, HTTPResponse):
              return self._cast(first, request, response)
          if isinstance(first, StringType):
              return itertools.chain([first], out)
          if isinstance(first, unicode):
              return itertools.imap(lambda x: x.encode(response.charset),
                                    itertools.chain([first], out))
          return self._cast(HTTPError(500, 'Unsupported response type: %s'\
                                           % type(first)), request, response)