remove MonadFail instances of ST

CLC proposal: https://github.com/haskell/core-libraries-committee/issues/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.

6 files changed, 7 insertions, 18 deletions

diff --git a/libraries/base/Control/Monad/ST/Lazy/Imp.hs b/libraries/base/Control/Monad/ST/Lazy/Imp.hs
index 888839132e..e67daf9dd1 100644
--- a/libraries/base/Control/Monad/ST/Lazy/Imp.hs
+++ b/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.
diff --git a/libraries/base/GHC/ST.hs b/libraries/base/GHC/ST.hs
index 98ba18b3d8..27749e83bb 100644
--- a/libraries/base/GHC/ST.hs
+++ b/libraries/base/GHC/ST.hs
@@ -26,7 +26,6 @@ module GHC.ST (
 
 import GHC.Base
 import GHC.Show
-import Control.Monad.Fail
 
 default ()
 
@@ -79,10 +78,6 @@ instance Monad (ST s) where
         (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 (<>)
 
diff --git a/libraries/base/changelog.md b/libraries/base/changelog.md
index a905e2d2f3..ed12f00737 100644
--- a/libraries/base/changelog.md
+++ b/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*
 
diff --git a/testsuite/tests/ghci/scripts/T4175.stdout b/testsuite/tests/ghci/scripts/T4175.stdout
index 7b7423ec6e..91a1fbb881 100644
--- a/testsuite/tests/ghci/scripts/T4175.stdout
+++ b/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)
diff --git a/testsuite/tests/monadfail/MonadFailErrors.hs b/testsuite/tests/monadfail/MonadFailErrors.hs
index 008a0b2c16..733f923a15 100644
--- a/testsuite/tests/monadfail/MonadFailErrors.hs
+++ b/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
diff --git a/testsuite/tests/monadfail/MonadFailErrors.stderr b/testsuite/tests/monadfail/MonadFailErrors.stderr
index 285e6fb4e0..ac2af1e10d 100644
--- a/testsuite/tests/monadfail/MonadFailErrors.stderr
+++ b/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’