It works like a charm,
thanks a lot Jonathan!
Adam
On Jan 29, 2008, at 10:26 PM, Jonathan Cast wrote:
On 29 Jan 2008, at 9:44 PM, Adam Smyczek wrote:
Hi,
My application has to manage a data set. I assume the state monad
is designed for this.
The state changes in functions that:
a. perform IO actions and
b. return execution status and execution trace (right now I'm
using WriteT for this).
Is the best solution:
1. to build a monad stack (for example State -> Writer -> IO) or
2. to use IORef for the data set or
3. something else?
Are monad stacks with 3 and more monads common?
I'd say they're fairly common, yes; at least, they don't jump out
at me as bad style (especially when the monads are fairly
orthogonal, as here).
How could an example implementation look like?
newtype Program alpha
= Program { runProgram :: StateT Config (WriterT [String] IO)
alpha }
deriving (Functor, Monad, MonadWriter, MonadState)
What I have for now is:
-- Status
data Status = OK | FAILED deriving (Show, Read, Enum)
-- Example data set manages by state
type Config = [String]
-- WriterT transformer
type OutputWriter = WriterT [String] IO Status
-- example execute function
execute :: [String] -> OutputWriter
execute :: [String] -> Program Status
execute fs = do
rs <- liftIO loadData fs
tell $ map show rs
return OK
-- run it inside e.g. main
(s, os) <- runWriterT $ execute files
(s', (s, os)) <- runWriterT (runStateT (runProgram $ execute files)
inputstate)
It's a bit tricky, since you have to write it inside-out, but it
should only type check if you've got it right :)
How do I bring a state into this, for example for:
execute fs = do
?? conf <- get ?? -- get Config from state
Right.
rs <- liftIO loadData conf fs
?? set conf ?? -- change Config and set to state
Right.
tell "new state:"
Right.
tell $ show conf
return OK
Do I have to use
Depends on what you mean by `have to'. If you don't want to thread
the state yourself, and you don't want to use an IORef, you'll need
some implementation of a state monad. That will have to be in the
form of a monad transformer applied to IO, so the easy answer is
`yes'.
and how do I use StateT in this context:
data DataState = StateT Config OutputWriter ??
This is parenthesized wrong; the output type goes outside the
parentheses around WriterT:
StateT Config (WriterT [String] IO) Status
not
StateT Config (WriterT [String] IO Status)
and how do I run it runStateT . runWriterT?
Other way 'round, as above.
HTH
jcc
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