betterShow.hs

{-# LANGUAGE ConstraintKinds #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# OPTIONS_GHC -fdefer-type-errors #-}
module Main where

import Data.Constraint
import Control.Exception
import System.IO.Unsafe

show' y = unsafePerformIO $ do
    res <- try $ evaluate $ show'' Dict y
    case res of
        Right x -> return x
        Left  (ErrorCall _) -> return "no show"



show'' :: (Dict (Show a)) -> a -> String
show'' Dict x = show x

t1 = show' 5
-- "5"

t2 = show' id
-- "no show"