Re: [Haskell-cafe] State Monad

3 Mar 2004

      Am Mittwoch, 3. März 2004 14:44 schrieb Georg Martius:
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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    
         ...
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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.
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Thanks!
  Georg
Wolfgang

Re: [Haskell-cafe] State Monad

Wolfgang Jeltsch