Warning found:
in module Main
at tests/purs/warning/NewtypeInstance3.purs:21:1 - 21:86 (line 21, column 1 - line 21, column 86)

  The derived newtype instance for
                                  
    Main.MonadWriter w0           
                     (MyWriter w0)
                                  
  does not include a derived superclass instance for Main.MonadTell.

in value declaration monadWriterMyWriter

where w0 is a rigid type variable
        bound at (line 0, column 0 - line 0, column 0)

See https://github.com/purescript/documentation/blob/master/errors/MissingNewtypeSuperclassInstance.md for more information,
or to contribute content related to this warning.