Haskell - Advanced Overlap

advancedOverlap.hs

{-# LANGUAGE DataKinds #-}
{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE PolyKinds #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE UndecidableInstances #-}

-- https://wiki.haskell.org/GHC/AdvancedOverlap
-- when using closed type family, data kinds, and proxy
module Main where

import Data.Proxy

class Print' flag a where
    print' :: Proxy flag -> a -> IO ()

class Print a where
    print :: a -> IO ()

-- Is there a way to avoid this explicitness?
type family ShowPred a where
  ShowPred Int    = 'True
  ShowPred Bool   = 'True
  ShowPred [a]   = ShowPred a
  ShowPred a     = 'False

instance Show a => Print' 'True a where
  print' _ x = putStrLn (show x)
instance Print' 'False a where
  print' _ _ = putStrLn "No show method"

instance (Print' (ShowPred a) a) => Print a where
    print a = print' (flag a) a
      where
        flag :: a -> Proxy (ShowPred a)
        flag _ = Proxy

See also https://kseo.github.io/posts/2017-02-05-avoid-overlapping-instances-with-closed-type-families.html
https://gist.github.com/phadej/cae76444548b9cffa10d9675e013847b