[Haskell-cafe] need help understanding how to specify constraints on monads
Dennis Raddle
dennis.raddle at gmail.com
Thu Jun 28 05:22:46 UTC 2018
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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