remove MonadFail instances of ST

CLC proposal: haskell/core-libraries-committee#33

The instances had `fail` implemented in terms of `error`, whereas the
idea of the `MonadFail` class is that the `fail` method should be
implemented in terms of the monad itself.

libraries/base/Control/Monad/ST/Lazy/Imp.hs

@@ -190,10 +190,6 @@ instance Monad (ST s) where
        in
          unST (k r) new_s
 
--- | @since 4.10
-instance MonadFail (ST s) where
-    fail s = errorWithoutStackTrace s
-
 -- | Return the value computed by an 'ST' computation.
 -- The @forall@ ensures that the internal state used by the 'ST'
 -- computation is inaccessible to the rest of the program.

libraries/base/GHC/ST.hs

@@ -26,7 +26,6 @@ module GHC.ST (
 
 import GHC.Base
 import GHC.Show
-import Control.Monad.Fail
 
 default ()
 
@@ -78,10 +77,6 @@ instance Monad (ST s) where
         case (k r) of { ST k2 ->
         (k2 new_s) }})
 
--- | @since 4.11.0.0
-instance MonadFail (ST s) where
-    fail s = errorWithoutStackTrace s
-
 -- | @since 4.11.0.0
 instance Semigroup a => Semigroup (ST s a) where
     (<>) = liftA2 (<>)

libraries/base/changelog.md

@@ -43,6 +43,11 @@
     CPtrdiff, CSize, CWchar, CSigAtomic, CLLong, CULLong, CBool, CIntPtr, CUIntPtr,
     CIntMax, CUIntMax.
 
+  * Remove instances of `MonadFail` for the `ST` monad (lazy and strict) as per
+    the [Core Libraries proposal](/~https://github.com/haskell/core-libraries-committee/issues/33).
+    A [migration guide](/~https://github.com/haskell/core-libraries-committee/blob/main/guides/no-monadfail-st-inst.md)
+    is available.
+
 
 ## 4.16.0.0 *Nov 2021*
 

testsuite/tests/ghci/scripts/T4175.stdout

@@ -41,10 +41,10 @@ type Maybe :: * -> *
 data Maybe a = Nothing | Just a
   	-- Defined in ‘GHC.Maybe’
 instance Traversable Maybe -- Defined in ‘Data.Traversable’
+instance MonadFail Maybe -- Defined in ‘Control.Monad.Fail’
 instance Foldable Maybe -- Defined in ‘Data.Foldable’
 instance Applicative Maybe -- Defined in ‘GHC.Base’
 instance Functor Maybe -- Defined in ‘GHC.Base’
-instance MonadFail Maybe -- Defined in ‘Control.Monad.Fail’
 instance Monad Maybe -- Defined in ‘GHC.Base’
 instance Semigroup a => Monoid (Maybe a) -- Defined in ‘GHC.Base’
 instance Semigroup a => Semigroup (Maybe a)

testsuite/tests/monadfail/MonadFailErrors.hs

@@ -37,13 +37,6 @@ io = do
 
 
 
-st :: ST s a
-st = do
-    Just x <- undefined
-    undefined
-
-
-
 reader :: r -> a
 reader = do
     Just x <- undefined

testsuite/tests/monadfail/MonadFailErrors.stderr

@@ -33,7 +33,7 @@ MonadFailErrors.hs:28:5: error:
             = do Just x <- undefined
                  undefined
 
-MonadFailErrors.hs:49:5: error:
+MonadFailErrors.hs:42:5: error:
     • No instance for (MonadFail ((->) r))
         arising from a do statement
         with the failable pattern ‘Just x’