[Haskell-cafe] Question on nested State monads.

Albert Y. C. Lai trebla at vex.net
Sun Dec 29 15:26:12 UTC 2013

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)

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