From https://github.com/johnjcamilleri/fst/pull/3
From: Xavier Dectot <xavier@wheredoibegin.fr>
Date: Sat, 22 Mar 2025 11:07:04 +0100
Subject: [PATCH] allow to build with mtl-2.3

--- a/FST/TransducerInterface.hs
+++ b/FST/TransducerInterface.hs
@@ -127,7 +127,7 @@ import qualified FST.LBFT as L
 import FST.ReversalT
 
 import Control.Exception (IOException, catch, try)
-import Control.Monad.Error
+import Control.Monad.Except
 
 -- | Construct a deterministic, epsilon-free, minimal transducer
 compile :: Ord a => RReg a -> Sigma a -> Transducer a
@@ -156,23 +156,23 @@ numberOfTransitions transducer = sum [ length (transitionList transducer s)
                                      | s <- states transducer]
 
 -- | Load a transducer from file
-load :: FilePath -> ErrorT String IO (Transducer String)
+load :: FilePath -> ExceptT String IO (Transducer String)
 load = fmap read . open
 
 -- | Save a transducer from file
-save :: FilePath -> Transducer String -> ErrorT String IO ()
+save :: FilePath -> Transducer String -> ExceptT String IO ()
 save file auto = saveToFile file (show auto)
 
 -- | Open a file and return contents as string
-open :: FilePath -> ErrorT String IO String
-open file = ErrorT $ catch 
-  (Right `liftM` readFile file) 
+open :: FilePath -> ExceptT String IO String
+open file = ExceptT $ catch 
+  (Right `fmap` readFile file) 
   (\(e :: IOException) -> return $ throwError $ "Error: Unable to open \"" ++ file ++ "\"")
 
 -- | Save contents (as string) to a file
-saveToFile :: FilePath -> String -> ErrorT String IO ()
-saveToFile file str = ErrorT $ catch 
-  (Right `liftM` writeFile file str) 
+saveToFile :: FilePath -> String -> ExceptT String IO ()
+saveToFile file str = ExceptT $ catch 
+  (Right `fmap` writeFile file str) 
   (\(e :: IOException) -> return $ throwError $ "Error: Unable to save to \"" ++ file ++ "\"")
 
 -- | The empty transucer
--- a/Main.hs
+++ b/Main.hs
@@ -209,9 +209,9 @@ import FST.Info
 
 import Text.Printf
 
-import Control.Monad (liftM)
+import Control.Monad (liftM, when, unless)
 import Control.Monad.State
-import Control.Monad.Error
+import Control.Monad.Except
 import Control.Monad.IO.Class (liftIO)
 import System.Environment (getArgs)
 import System.Console.Haskeline
@@ -225,7 +225,7 @@ main = do
       welcome
       runInputT defaultSettings (evalStateT loop emptyInfo)
     as -> do
-      ret <- runErrorT (batchMode as)
+      ret <- runExceptT (batchMode as)
       case ret of
         Left err -> putStrLn err
         Right _  -> return ()
@@ -246,9 +246,9 @@ welcome = putStr $ unlines [
   ]
 
 -- | Run in batch mode with given arguments
-batchMode :: [String] -> ErrorT String IO ()
+batchMode :: [String] -> ExceptT String IO ()
 batchMode cmdopt = do
-  (file, cmd) <- (ErrorT . return . parseBatch) cmdopt
+  (file, cmd) <- (ExceptT . return . parseBatch) cmdopt
 
   -- Only accept .NET or .FST files
   when (not (isNET file) && not (isFST file)) $ 
@@ -256,7 +256,7 @@ batchMode cmdopt = do
 
   transducer <- if isFST file
       then do str <- open file
-              fmap (flip compile []) $ ErrorT $ return $ parseProgram str
+              fmap (flip compile []) $ ExceptT $ return $ parseProgram str
       
       -- Load transducer directly from .NET files
       else load file
@@ -319,14 +319,14 @@ loop = do
     Just Quit -> lift $ outputStrLn "Session ended."
     Just cmd  -> do
       -- Run a single command entered at the prompt
-      result <- runErrorT $ runCmd cmd
+      result <- runExceptT $ runCmd cmd
 
       -- Print the resulting output or error message
       lift (either outputStrLn outputStrLn result)
       loop 
 
 -- | Called for each user command
-runCmd :: InteractiveCommand -> ErrorT String (StateT Info (InputT IO)) String
+runCmd :: InteractiveCommand -> ExceptT String (StateT Info (InputT IO)) String
 runCmd  BuildTransducer = do
   info <- get
   unless (expressionRead info) $ throwError noExpression
@@ -372,19 +372,19 @@ runCmd ViewTransducer = do
 
 runCmd (Load file)
   | isFST file = do
-    res <- liftIO $ runErrorT $ open file
-    str <- ErrorT $ return res
-    reg <- ErrorT $ return $ parseProgram str
+    res <- liftIO $ runExceptT $ open file
+    str <- ExceptT $ return res
+    reg <- ExceptT $ return $ parseProgram str
     modify (updateExpression reg)
     return (printf "Loaded a regular relation from %s." file)
   | isNET file = do
-    res    <- liftIO $ runErrorT $ load file
-    transd <- ErrorT $ return res
+    res    <- liftIO $ runExceptT $ load file
+    transd <- ExceptT $ return res
     modify (updateTransducer transd)
     return (printf "Loaded transducer from file %s." file)
   | isDAT file = do
-    res <- liftIO $ runErrorT $ open file
-    str <- ErrorT $ return res
+    res <- liftIO $ runExceptT $ open file
+    str <- ExceptT $ return res
     modify $ updateInput $ words str
     return $ printf "Read input from file %s." file
   | otherwise =
@@ -392,10 +392,10 @@ runCmd (Load file)
   
 runCmd (LUnion file1 file2)
   | isNET file1 && isNET file2 = do
-    res1 <- liftIO $ runErrorT $ load file1
-    res2 <- liftIO $ runErrorT $ load file2
-    t1 <- ErrorT $ return res1
-    t2 <- ErrorT $ return res2
+    res1 <- liftIO $ runExceptT $ load file1
+    res2 <- liftIO $ runExceptT $ load file2
+    t1 <- ExceptT $ return res1
+    t2 <- ExceptT $ return res2
     modify $ updateTransducer (unionT t1 t2)
     return "Loaded and unified two transducers."
                                           
@@ -404,18 +404,18 @@ runCmd (LUnion file1 file2)
   | otherwise = return $ printf "Unable to union %s and %s." file1 file2 where
   unionWith file = do
     info <- get
-    res <- liftIO $ runErrorT $ load file
+    res <- liftIO $ runExceptT $ load file
     unless (transducerBuilt info) $ throwError "No interior transducer built."
-    r1 <- ErrorT $ return res
+    r1 <- ExceptT $ return res
     modify $ \info -> updateTransducer (unionT r1 (getTransducer info)) info
     return "Loaded a transducer, and unified it with the interior transducer."                
 
 runCmd (LProduct file1 file2)
   | isNET file1  && isNET file2 = do
-    res1 <- liftIO $ runErrorT $ load file1
-    res2 <- liftIO $ runErrorT $ load file2
-    t1 <- ErrorT (return res1)
-    t2 <- ErrorT (return res2)
+    res1 <- liftIO $ runExceptT $ load file1
+    res2 <- liftIO $ runExceptT $ load file2
+    t1 <- ExceptT (return res1)
+    t2 <- ExceptT (return res2)
     modify (updateTransducer (productT t1 t2))
     return "Loaded and concatenated two transducers."
   | isNET file1 && isTHIS file2 = productWith file1
@@ -423,16 +423,16 @@ runCmd (LProduct file1 file2)
   | otherwise = return $ printf "Unable to concatenate %s and %s." file1 file2 where
   productWith file = do
     info <- get
-    res <- liftIO $ runErrorT $ load file
+    res <- liftIO $ runExceptT $ load file
     unless (transducerBuilt info) $ throwError "No interior transducer built."
-    t1 <- ErrorT $ return res
+    t1 <- ExceptT $ return res
     modify $ \info -> updateTransducer (productT t1 (getTransducer info)) info
     return "Loaded a transducer, and concatenated it with the interior transducer."
 
 runCmd (LStar file)
   | isNET file = do
-    res <- liftIO $ runErrorT $ load file
-    t1 <- ErrorT (return res)
+    res <- liftIO $ runExceptT $ load file
+    t1 <- ExceptT (return res)
     modify $ updateTransducer (starT t1)
     return "Loaded a transducer, and applied Kleene's star."
   | isTHIS file = do
@@ -444,10 +444,10 @@ runCmd (LStar file)
 
 runCmd (LComposition file1 file2)
   | isNET file1  && isNET file2 = do
-    res1 <- liftIO $ runErrorT $ load file1
-    res2 <- liftIO $ runErrorT $ load file2
-    t1 <- ErrorT (return res1)
-    t2 <- ErrorT (return res2)
+    res1 <- liftIO $ runExceptT $ load file1
+    res2 <- liftIO $ runExceptT $ load file2
+    t1 <- ExceptT (return res1)
+    t2 <- ExceptT (return res2)
     modify $ updateTransducer (compositionT t1 t2)
     return "Loaded and composed two transducers."
   | isNET file1  && isTHIS file2 = composeWith file1
@@ -455,9 +455,9 @@ runCmd (LComposition file1 file2)
   | otherwise = return $ printf "Unable to compose %s and %s." file1 file2 where
   composeWith file = do
     info <- get
-    res <- liftIO $ runErrorT $ load file
+    res <- liftIO $ runExceptT $ load file
     unless (transducerBuilt info) $ throwError "No interior transducer built."
-    t1 <- ErrorT (return res)
+    t1 <- ExceptT (return res)
     modify $ \info -> updateTransducer (compositionT t1 (getTransducer info)) info
     return "Loaded a transducer, and composed it with the interior transducer."
 
@@ -465,12 +465,12 @@ runCmd (Save file) = do
   info <- get
   case () of
     _ | isNET file -> do
-        res <- liftIO $ runErrorT $ save file $ getTransducer info
-        _ <- ErrorT (return res)
+        res <- liftIO $ runExceptT $ save file $ getTransducer info
+        _ <- ExceptT (return res)
         return $ printf "Saved transducer to file %s." file
       | outputsRead info -> do
-        res <- liftIO $ runErrorT $ saveToFile file $ unlines $ getOutputs info
-        _ <- ErrorT (return res)
+        res <- liftIO $ runExceptT $ saveToFile file $ unlines $ getOutputs info
+        _ <- ExceptT (return res)
         return $ printf "Saved outputs to file %s." file
       | otherwise -> return noOutputs