scalaz/scalaz — pull request #160 by romanb (2012-09-28 21:42:12+00:00)
Title: MonadTrans instance for WriterT

While playing with Reader(T)/Writer(T) combinations I couldn't find a MonadTrans instance for WriterT so I added one. I also made the Reader/ReaderT types covariant in A so they're more compatible with WriterT and one can do:

type MyReaderT[+A] = ReaderT[Id, MyEnv, A]
type MyWriterT[+W, +A] = WriterT[MyReaderT, W, A]