injection experiment

InjectionExperiment.hs

{-# LANGUAGE MultiParamTypeClasses, TypeFamilies,
             GeneralizedNewtypeDeriving, DeriveGeneric, 
             ConstraintKinds, FunctionalDependencies, UndecidableSuperClasses, RankNTypes, TypeApplications, ScopedTypeVariables, 
             DefaultSignatures, FlexibleInstances #-}

module Main where

import Data.Proxy
import Control.Monad.IO.Class
import Control.Monad.State.Strict
import GHC.Generics
import Control.Newtype
import Lens.Micro
import Lens.Micro.Mtl

class Monad n => DBState n s where
    save' :: Int -> StateT s n ()
    load' :: StateT s n Int

class DB m where
    save :: Int -> m ()
    default save :: forall n e e'.(Newtype (m ()), O (m ()) ~ StateT e n (),Has (DBState n) e e') => Int -> m () 
    save i = pack $ zoom (getLens (Proxy @(DBState n))) $ save' i
    load :: m Int  
    default load :: forall n e e'.(Newtype (m Int), O (m Int) ~ StateT e n Int,Has (DBState n) e e') => m Int
    load = pack $ zoom (getLens (Proxy @(DBState n))) $ load'

--
data DummyDBState = DummyDBState

instance MonadIO m => DBState m DummyDBState where
    save' = undefined
    load' = undefined

--
--class Has a b where
--    getLens :: Lens' b a

-- The component is determined by the constraint and the global record type.
class c b => Has c a b | c a -> b where
    getLens :: Proxy c -> Lens' a b
    
--
instance MonadIO n => Has (DBState n) ((),DummyDBState) DummyDBState where
    getLens _ = _2

-- The idea is being able to standalone derive DB for newtypes over StateT,
-- when a component of the state has a suitable DBState instance.
newtype Foo r = Foo { getFoo :: StateT ((),DummyDBState) IO r } deriving (Functor,Applicative,Monad,Generic)
instance Newtype (Foo r)
instance DB Foo

main :: IO ()
main = putStrLn "Hello, Haskell!"

package.cabal

-- Initial injecty.cabal generated by cabal init.  For further
-- documentation, see http://haskell.org/cabal/users-guide/

name:                injecty
version:             0.1.0.0
-- synopsis:
-- description:
license:             BSD3
license-file:        LICENSE
author:              daniel
maintainer:          daniel@bogusemailserver.com
-- copyright:
-- category:
build-type:          Simple
extra-source-files:  ChangeLog.md
cabal-version:       >=1.10

executable injecty
  main-is:             Main.hs
  -- other-modules:
  -- other-extensions:
  build-depends:       base >=4.10 && <4.11,
                       transformers >= 0.5.5.0,
                       mtl >= 2.2.1,
                       microlens >= 0.4.8.1,
                       microlens-mtl >= 0.1.11.0,
                       newtype-generics >= 0.5.1
  -- hs-source-dirs:
  default-language:    Haskell2010