Hi,
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) ()
modifyT trans
= do s <- lift get
t <- get
let (t', s') = trans (t, s)
lift (put s')
put t'
now I can use either
let str' = withString str $
do modify $ foo_stringtrans
modify $ bar_stringtrans
or
let str' = withStringAndT str (gen) $
do modifyT $ foo_stringgentrans
lift $ modify $ foo_stringtrans
Cheers,
Georg
On Thu, 04 Mar 2004 08:51:04 +1300, Tom Pledger
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.
[...]
Hi.
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?
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.
Regards, Tom