mutate leaves of tree

mut.hs

{-# LANGUAGE ScopedTypeVariables, DeriveFunctor, DeriveTraversable,
  DeriveFoldable #-}

import Data.Foldable (traverse_)
import qualified Data.IORef as IORef
import Data.IORef (IORef)

data AST ident
  = Var ident
  | Lam ident
        (AST ident)
  | App (AST ident)
        (AST ident)
  deriving (Functor, Traversable, Foldable, Show)

idExpr :: AST String
idExpr = Lam "x" (Var "x")

toMut :: AST ident -> IO (AST (IORef ident))
toMut (Var ident) = Var <$> IORef.newIORef ident
toMut (Lam ident rest) = Lam <$> IORef.newIORef ident <*> toMut rest
toMut (App ast1 ast2) = App <$> (toMut ast1) <*> (toMut ast2)

renameStuff :: (String -> String) -> AST (IORef String) -> IO ()
renameStuff f ast = traverse_ (flip IORef.modifyIORef f) ast

main :: IO ()
main = do
  mutAst <- toMut idExpr
  renameStuff ("Sup_" ++) mutAst
  pureAst <- traverse IORef.readIORef mutAst
  putStrLn $ show $ pureAst