[Haskell-cafe] State Monad

[Wolfgang Jeltsch]

Am Mittwoch, 3. März 2004 14:44 schrieb Georg Martius:
> [...]

> Now I have also functions to map from (a, String) -> (a,String).  I could
> write:
>
> modifyT :: ((a, String) -> (a, String)) -> a ->  State String a
> modifyT trans a = do str <- get
>                      let (a', str') = trans (a, str)
>                      put str'
>                      return a'
>
> f :: State String ()
> f = do put "hallo"
>          modify strTrans
>          i <- modifyT strIntTrans 4
>              -- strIntTrans :: (Int, String) -> (Int, String)
>          i' <- modifyT strIntTrans i    
>          ...

> But this is obviously awkward.  How can I stick two Monads in each other? I
> could't figure out how to use StateT.

StateT is one solution.  See
    http://www.haskell.org/pipermail/haskell/2004-January/013330.html
and the follow-ups (available via the "next message" links).  With StateT, 
modifyT may be written as follows (untested code):
    modifyT ::  ((a, String) -> (a, String)) -> StateT a (State String) ()
    modifyT trans
        = do 
              str <- liftM get
              a <- get
              let (a', str') = trans (a, str)
              liftM (put str')
              put a'

Alternatively, you can use the ordinary state monad with (a,String) as its 
state.  Then modifyT is just modify.

> [...]

> Thanks!
>   Georg

Wolfgang