How to redefine functions in Haskell, based on Hillel's article here: https://buttondown.email/hillelwayne/archive/i-am-disappointed-by-dynamic-typing/

MutDef.hs

{-# language BangPatterns #-}

module MutDef where

import Data.IORef
import System.IO.Unsafe
import Data.Map (Map)
import qualified Data.Map as Map
import GHC.Exts

{-# NOINLINE doubleImpl #-}
doubleImpl :: IORef (Int -> Int)
doubleImpl = unsafePerformIO $ do
    newIORef (\x -> x * 2)

double :: Int -> Int
double i = unsafePerformIO $ do
    impl <- readIORef doubleImpl
    pure (impl i)
{-# NOINLINE double #-}

g :: Int -> Int
g x = double (double x) + double x

main = do
    print (g 2)
    -- 12
    calledArgs <- newIORef mempty :: IO (IORef (Map Int Int))

    modifyIORef' doubleImpl $ \f !i ->
        unsafePerformIO $ do
            modifyIORef' calledArgs (Map.insertWith (+) i 1)
            pure (f i)

    print (g 2)
    -- 12

    print =<< readIORef calledArgs
    -- fromList [(2,1),(4,1)]

redefineDouble :: ((Int -> Int) -> Int -> Int) -> IO ()
redefineDouble withOldFn = 
    modifyIORef' doubleImpl $ \f !i ->