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.

3 files changed, 5 insertions, 9 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*