Question on nested State monads.
Hi all, 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? Thanks, -db
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
On 13-12-28 09:24 PM, David Banas wrote:
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?
I do not know what the whole sentence "must I ... the outer one" is talking about. But why don't you go scientific and use concrete experiments to find out? Here is a starter: import Control.Monad.State main = print answer answer :: (Bool, Char) answer = evalState (evalStateT program True) 't' program :: StateT Bool (State Char) (Bool, Char) program = do bool <- get char <- lift get return (bool, char)
participants (3)
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Albert Y. C. Lai -
David Banas -
lucas di cioccio