[Haskell-cafe] State Monad
mai99dgf at studserv.uni-leipzig.de
Thu Mar 4 11:08:05 EST 2004
thanks for your suggestion. The thing is, that I don't want to change the
type of my transformation functions.
To answer Iavor's question: I have basically two types of transformation
functions. One StringTransformation (String -> String) and one
transformation with a string and something (e.g. random generator)
((a,String) -> (a,String)). The vast majority of transformation I use are
from the first type. I was looking for a nice way to write this in a the
compact style. I haven't thought about exceptions yet.
My current approach is as follows:
withString :: String -> State String () -> String
withString state monad = execState monad state
withStringAndT :: String -> t -> StateT t (State String) () -> String
withStringAndT state t monad = execState (execStateT monad t) state
modifyT :: ((t, String) -> (t, String))
-> StateT t (State String) ()
= do s <- lift get
t <- get
let (t', s') = trans (t, s)
lift (put s')
now I can use either
let str' = withString str $
do modify $ foo_stringtrans
modify $ bar_stringtrans
let str' = withStringAndT str (gen) $
do modifyT $ foo_stringgentrans
lift $ modify $ foo_stringtrans
On Thu, 04 Mar 2004 08:51:04 +1300, Tom Pledger <tpledger at ihug.co.nz>
> Georg Martius wrote:
>> 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.
> People have already replied about the state monad aspect, but there's
> another small improvement I'd like to suggest.
> Look at what modifyT does with 'trans' and 'a'. They are always used
> together. So, how about combining them *outside* the definition of
> modifyT :: (String -> (a, String)) -> State String a
> modifyT trans = do (a, s) <- gets trans
> put s
> return a
> f = do ...
> i <- modifyT (strIntTrans 4) -- strIntTrans :: Int ->
> String -> (Int, String)
> i' <- modifyT (strIntTrans i)
> Aside: if you rewrite ($) similarly, you get id.
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