Translation of the PrimMonad class to a Backpack signature with example modules that implement it.

PrimMonad.bkp

-- vi:syntax=haskell
{-# LANGUAGE UnboxedTuples, MagicHash, TypeFamilies, StandaloneKindSignatures, RankNTypes, AllowAmbiguousTypes, ConstraintKinds, UndecidableInstances #-}
unit primmonad-indef where
  signature PrimMonad where
    import Data.Kind
    import GHC.Prim
    type C :: (* -> *) -> * -> Constraint
    type family C m s
    type PrimMonad m s = (Monad m, C m s)
    primitive :: PrimMonad m s => (State# s -> (# State# s, a #)) -> m a

  module PrimReaderT where
    import Data.Kind
    import Control.Monad.Trans.Reader
    import Control.Monad.Trans.Class
    import GHC.Prim
    import qualified PrimMonad as Super

    type C :: (* -> *) -> * -> Constraint
    type family C m s
    type instance C (ReaderT _ m) s = (Super.PrimMonad m s)
    type PrimMonad m s = (Monad m, C m s)
    primitive :: (m ~ ReaderT r n, PrimMonad m s) => (State# s -> (# State# s, a #)) -> m a
    primitive = lift . Super.primitive
    
  module Test (primitive) where
    -- import PrimMonad
    import PrimReaderT

unit primmonad-st where
  module PrimST where
    import Data.Kind
    import GHC.ST
    import GHC.Prim

    type C :: (* -> *) -> * -> Constraint
    type family C m s
    type instance C m s = m ~ ST s
    type PrimMonad m s = (Monad m, C m s)
    primitive :: PrimMonad m s => (State# s -> (# State# s, a #)) -> m a
    primitive = ST

unit primmonad-io where
  module PrimIO where
    import Data.Kind
    import GHC.IO
    import GHC.Prim

    type C :: (* -> *) -> * -> Constraint
    type family C m s
    type instance C m s = (m ~ IO, s ~ RealWorld)
    type PrimMonad m s = (Monad m, C m s)
    primitive :: PrimMonad m s => (State# s -> (# State# s, a #)) -> m a
    primitive = IO

unit main where
  -- dependency primmonad-indef[PrimMonad=primmonad-io:PrimIO]
  dependency primmonad-indef[PrimMonad=primmonad-st:PrimST]

  module Main where
    import Test
    test = primitive :: _
    main = putStrLn "Hello, World!"