[Haskell] signification of RWS and RWST

[Wolfgang Jeltsch]

Hello,

why do we have RWS and RWST in the libaries?  Isn't RWS r w s a isomorphic to 
ReaderT r (StateT s (Writer w a)) and RWST r w s m isomorphic to
ReaderT r (StateT s (WriterT w m a))?

Best wishes,
Wolfgang