module Foo (go) where
import Control.Monad.Writer.Strict
import Data.Monoid (Sum(..))
type Reg = Double

data S1 = S1
data S2 = S2 Double

addReg :: Double -> Writer (Sum Reg) ()
addReg = tell . Sum

f :: S1 -> Double -> Writer (Sum Reg) (S1,Double)
f S1 i = do
  addReg $ max 0.1 i
  pure (S1,i)

g :: S2 -> Double -> Writer (Sum Reg) (S2,Double)
g (S2 x) i = do
  addReg $ max x i
  pure (S2 (x+i),sin (x/i))

go :: (((S1, S2), Double), Reg)
go = fmap getSum $ runWriter $ do
 let i = 1 :: Double
 let (s1,s2) = (S1,S2 0)
 (s1',o1) <- f s1 i
 (s2',o2) <- g s2 (min o1 i)
 pure ((s1',s2'),o2)