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?
How could an example implementation look like?
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 fs = do
rs <- liftIO loadData fs
tell $ map show rs
return OK
-- run it inside e.g. main
(s, os) <- runWriterT $ execute files
How do I bring a state into this, for example for:
execute fs = do
?? conf <- get ?? -- get Config from state
rs <- liftIO loadData conf fs
?? set conf ?? -- change Config and set to state
tell "new state:"
tell $ show conf
return OK
Do I have to use and how do I use StateT in this context:
data DataState = StateT Config OutputWriter ??
and how do I run it runStateT . runWriterT?
Thanks for help,
Adam
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