how to converting function to cps recursively

Question

i'd like to define cpsRec as follows but i couldn't.

please let me know if you come up with ideas for implementation.

import Control.Monad.Trans.Cont (Cont)

type family ContRec r x where
  ContRec r (a -> b) = a -> ContRec r b
  ContRec r a = Cont r a

cpsRec :: (a -> b) -> (a -> ContRec r b)
cpsRec f a =
  let fa = f a
   in case fa of
        (x -> y) -> cpsRec fa -- error!
        _ -> pure fa -- error!

-- use case
addT :: Int -> Int -> Int -> Int
addT x y z = x + y + z

addCpsT :: Int -> Int -> Int -> Cont r Int
addCpsT = cpsRec addT

Answer 1

Here is an example of implementation of cpsRec which works for a function with any number of arguments:

{-# LANGUAGE DataKinds             #-}
{-# LANGUAGE FlexibleInstances     #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE ScopedTypeVariables   #-}
{-# LANGUAGE TypeFamilies          #-}
{-# LANGUAGE UndecidableInstances  #-}

import Control.Monad.Trans.Cont (Cont)
import Data.Proxy (Proxy(..))

-- | Helper type function to distinguish function from non-function
type family IsFun a where
  IsFun (a -> b) = 'True
  IsFun a = 'False

-- | Helper type class which includes auxiliary lifted Bool type parameter
class GContRec (i :: Bool) a rs where
  gcpsRec :: Proxy i -> a -> rs

-- | Intermediate recursive case: for a function `a -> b` (when `IsFun == True`)
instance (GContRec (IsFun b) b rs', (a -> rs') ~ rs) => GContRec 'True (a -> b) rs where
  gcpsRec _ f = gcpsRec (Proxy :: Proxy (IsFun b)) . f

-- | Base recursive case: not a function (`IsFun == False`) i.e. last argument - lift it to `Cont t a`
instance GContRec 'False a (Cont r a) where
  gcpsRec _ = pure

-- | Type class which defines very "generic" `cpsRec` without auxiliary type parameter
class ContRec a rs where
  cpsRec :: a -> rs

-- | Our implementation of `cpsRec` for `Cont`
instance (GContRec (IsFun a) a rs) => ContRec a rs where
  cpsRec = gcpsRec (Proxy :: Proxy (IsFun a))


-- Works for functions with any number of arguments
notCpsT :: Bool -> Cont r Bool
notCpsT = cpsRec not 

addT :: Int -> Int -> Int -> Int
addT x y z = x + y + z

addCpsT :: Int -> Int -> Int -> Cont r Int
addCpsT = cpsRec addT

foldrCpsT :: Int -> [Int] -> Cont r Int
foldrCpsT = cpsRec (foldr (+))