Your sample code has a few bugs which make it not compile, for example the following is not valid syntax data MyStateData t1 t2 = MyStateData t1 t2 { theGen :: StdGen , theReports :: [StepReport t1 t2] } and you use "StepReport" when I think you mean "ReportData". Could you post a version which is completely working besides the error you are trying to solve? Otherwise it's rather hard to help. Tom On Wed, Jun 27, 2018 at 10:22:46PM -0700, Dennis Raddle wrote:
I'm writing a program with several functions, some of which depend on certain fields in a state monad, others of which depend on others, but no routine needs all the fields.
So I thought I would declare a two classes, one for each type of data need that a function has:
-- as an aside, here's an example of data which is parameterized by two types.
data ReportData t1 t2 = ...
-- this is rolling my own state monad with a random generator class Monad m => RandMonad m where getGen :: m StdGen putGen :: StdGen -> ()
-- this is a class of state monad which logs ReportData:
class Monad m => LogMonad m where putReport :: ReportData t1 t2 -> m ()
For a particular use case, I declare a type of State monad:
data MyStateData t1 t2 = MyStateData t1 t2 { theGen :: StdGen , theReports :: [StepReport t1 t2] }
type MyState t1 t2 = State (MyStateData t1 t2)
And I try to define my instances:
instance RandMonad (MyState t1 t2) where getGen = gets theGen putGen g = modify (\s -> s { theGen = g})
instance LogMonad (MyState t1 t2) where putReport r = modify (\s -> s { theReports = r : theReports s})
I get an error on the LogMonad instance, saying that there's no instance for (MonadState (MyState t1 t2) (StateT (MyState t1 t2) Identity))
I guess I don't really understand typeclasses once you start using higher kinded types, so please enlighten me. Any reading on this subject would be helpful, too.
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