Re: [Haskell-cafe] Question on nested State monads.

Hi David,

If I want to use a second, nested State monad inside another State monad, must I thread the outer monad’s state through the processing of the inner one, even if the inner one falls out of scope before I return from the processing body of the outer one?

In short: yes you can nest StateT and to access the "inner parts" you need to lift the get/put/modify functions. The Real-World Haskell book has an example for stacking monad transformers of a same TypeClass (cf. http://book.realworldhaskell.org/read/monad-transformers.html "When explicit lifting is necessary".) The first time, it's best to convince yourself by writing down the types of lifted put/get. Best, --Lucas