Re: [Haskell-cafe] need help understanding how to specify constraints on monads

A compiled example where you can taste value level selection of aspect. https://lpaste.net/57731722032185344 - p On Thu, 28 Jun 2018 at 15:28, Paolino wrote:

All you need is the state to have different "aspects"

You can express this with parameters to your stateful computations

Here's an (uncompiled) sketch covering part of your case, using lenses

{-# language TemplateHaskell #-}

import Control.Lens (Lens', set, makeLenses) import System.Random import Control.Monad.State

-- a user of any state 's' which has a StdGen aspect, see the 's' is free here so putGen is polymorphic in it putGen :: Lens' s StdGen -> StdGen -> State s () putGen l g = modify $ set l g -- or for shorter see Control.Lens.Setter -- putGen = (.=)

... ...

-- a state as a record with all aspects data S = S { .... :: .... , _generator :: StdGen ..... :: .. }

makeLenses ''S -- automatically derive the lenses for you (generator function in example) -- equivalent at least to something like -- generator f g s = (\g' -> s{_generator = g'}) <$> f g

main = do g0 <- newStdGen print $ evalState (putGen generator g >> ....) $ S ... g0 ..

(This leaves you the burden of passing lenses around (one for each aspect) which you could alleviate with different techniques, if this is ever a concern)

As Tom said, typeclasses are not that good for this cases as it might seem at first glance

HTH

Best

paolino

On Thu, 28 Jun 2018 at 07:23, 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. _______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post.