Warning found:
in module Main
at tests/purs/warning/NewtypeInstance4.purs:23:1 - 23:86 (line 23, column 1 - line 23, column 86)

  The derived newtype instance for
                                  
    Main.MonadWriter w0           
                     (MyWriter w0)
                                  
  implies an superclass instance for Main.MonadTell which could not be verified.

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/UnverifiableSuperclassInstance.md for more information,
or to contribute content related to this warning.