Monad Transformers Step by Step

Monad Transformers Step by Step

As described:

It is not a paper about implementing transformers,
but about using them to write elegant, clean and
powerful programs in Haskell.

you wrote fluently, i read quickly,
pin pin pon pon bon bon, wow, it's so easy and simple.

but why? ...
how did that type check... orz
i should try myself, and use :t to see the types there.

All About Monads is a good reference,
but a little too hard (abstract?) for a beginner
who didn't familiar with monads.