Control/Monad/Trans/Class.hs

@@ -5,7 +5,7 @@
 #if __GLASGOW_HASKELL__ >= 710 && __GLASGOW_HASKELL__ < 802
 {-# LANGUAGE AutoDeriveTypeable #-}
 #endif
-#if __GLASGOW_HASKELL__ >= 808
+#if __GLASGOW_HASKELL__ >= 806
 {-# LANGUAGE QuantifiedConstraints #-}
 #endif
 -----------------------------------------------------------------------------
@@ -58,14 +58,29 @@ module Control.Monad.Trans.Class (
 --
 -- * @'lift' (m >>= f) = 'lift' m >>= ('lift' . f)@
 --
--- Since 0.6.0.0 and for GHC 8.8 and later, the requirement that @t m@
+-- Since 0.6.0.0 and for GHC 8.6 and later, the requirement that @t m@
 -- be a 'Monad' is enforced by the implication constraint
 -- @forall m. 'Monad' m => 'Monad' (t m)@ enabled by the
 -- @QuantifiedConstraints@ extension.
-#if __GLASGOW_HASKELL__ >= 808
+--
+-- === __Ambiguity error with GHC 9.0 to 9.2.2__
+-- These versions of GHC have a bug
+-- (<https://gitlab.haskell.org/ghc/ghc/-/issues/20582>)
+-- which causes constraints like
+--
+-- @
+-- (MonadTrans t, forall m. Monad m => Monad (t m)) => ...
+-- @
+--
+-- to be reported as ambiguous.  For transformers 0.6 and later, this can
+-- be fixed by removing the second constraint, which is implied by the first.
+#if __GLASGOW_HASKELL__ >= 806
 class (forall m. Monad m => Monad (t m)) => MonadTrans t where
 #else
--- prior to GHC 8.8 (base-4.13), the Monad class included fail
+-- Prior to GHC 8.8 (base-4.13), the Monad class included fail.
+-- GHC 8.6 (base-4.12) has MonadFailDesugaring on by default, so there
+-- is no need for users defining monad transformers to define fail in
+-- the Monad instance of the transformed monad.
 class MonadTrans t where
 #endif
     -- | Lift a computation from the argument monad to the constructed monad.
@@ -108,16 +123,23 @@ specialized lifting combinators, called @lift@/Op/
 
 {- $strict
 
-A monad is said to be /strict/ if its '>>=' operation is strict in its first
-argument.  The base monads 'Maybe', @[]@ and 'IO' are strict:
+A monad is said to be /strict/ if its '>>=' operation (and therefore also
+'>>') is strict in its first argument.  The base monads 'Maybe', @[]@
+and 'IO' are strict:
 
->>> undefined >> return 2 :: Maybe Integer
+>>> undefined >> Just 2
+*** Exception: Prelude.undefined
+>>> undefined >> [2]
+*** Exception: Prelude.undefined
+>>> undefined >> print 2
 *** Exception: Prelude.undefined
 
-However the monad 'Data.Functor.Identity.Identity' is not:
+However the monads 'Data.Functor.Identity.Identity' and @(->) a@ are not:
 
->>> runIdentity (undefined >> return 2)
-2
+>>> undefined >> Identity 2
+Identity 2
+>>> (undefined >> (+1)) 5
+6
 
 In a strict monad you know when each action is executed, but the monad
 is not necessarily strict in the return value, or in other components

Control/Monad/Trans/State/Strict.hs

@@ -291,6 +291,10 @@ modify f = state $ \ s -> ((), f s)
 -- new state.
 --
 -- * @'modify'' f = 'get' >>= (('$!') 'put' . f)@
+--
+-- Note that this is only strict in the top level of the state.
+-- Lazy components of the state will not be evaluated unless @f@
+-- evaluates them.
 modify' :: (Monad m) => (s -> s) -> StateT s m ()
 modify' f = do
     s <- get

changelog

 -*-change-log-*-
 
+0.6.0.6 Ross Paterson <R.Paterson@city.ac.uk> Jan 2023
+	* Fix for GHC 8.6
+
+0.6.0.5 Ross Paterson <R.Paterson@city.ac.uk> Jan 2023
+	* Revert to allowing MonadTrans constraint with GHC >= 8.6
+
 0.6.0.4 Ross Paterson <R.Paterson@city.ac.uk> Feb 2022
 	* Restrict deriving (Generic) to GHC >= 7.4
 

transformers.cabal

 name:         transformers
-version:      0.6.0.4
+version:      0.6.0.6
 license:      BSD3
 license-file: LICENSE
 author:       Andy Gill, Ross Paterson