base: Remove `Monad(fail)` method and reexport `MonadFail(fail)` instead

As per https://prime.haskell.org/wiki/Libraries/Proposals/MonadFail

Coauthored-by: Ben Gamari <ben@well-typed.com>

8 files changed, 4 insertions, 282 deletions

diff --git a/testsuite/tests/monadfail/MonadFailErrors.hs b/testsuite/tests/monadfail/MonadFailErrors.hs
index f9db31e5a0..008a0b2c16 100644
--- a/testsuite/tests/monadfail/MonadFailErrors.hs
+++ b/testsuite/tests/monadfail/MonadFailErrors.hs
@@ -1,8 +1,6 @@
 -- Test purpose:
 -- Break properly if MonadFail is live
 
-{-# LANGUAGE MonadFailDesugaring #-}
-
 module MonadFailWarnings where
 
 import Control.Monad.Fail
diff --git a/testsuite/tests/monadfail/MonadFailErrors.stderr b/testsuite/tests/monadfail/MonadFailErrors.stderr
index 27fc3c3cf5..285e6fb4e0 100644
--- a/testsuite/tests/monadfail/MonadFailErrors.stderr
+++ b/testsuite/tests/monadfail/MonadFailErrors.stderr
@@ -1,12 +1,12 @@
 
-MonadFailErrors.hs:16:5: error:
+MonadFailErrors.hs:14:5: error:
     • Could not deduce (MonadFail m)
         arising from a do statement
         with the failable pattern ‘Just x’
       from the context: Monad m
         bound by the type signature for:
                    general :: forall (m :: * -> *) a. Monad m => m a
-        at MonadFailErrors.hs:14:1-25
+        at MonadFailErrors.hs:12:1-25
       Possible fix:
         add (MonadFail m) to the context of
           the type signature for:
@@ -20,7 +20,7 @@ MonadFailErrors.hs:16:5: error:
             = do Just x <- undefined
                  undefined
 
-MonadFailErrors.hs:30:5: error:
+MonadFailErrors.hs:28:5: error:
     • No instance for (MonadFail Identity)
         arising from a do statement
         with the failable pattern ‘Just x’
@@ -33,7 +33,7 @@ MonadFailErrors.hs:30:5: error:
             = do Just x <- undefined
                  undefined
 
-MonadFailErrors.hs:51:5: error:
+MonadFailErrors.hs:49:5: error:
     • No instance for (MonadFail ((->) r))
         arising from a do statement
         with the failable pattern ‘Just x’
diff --git a/testsuite/tests/monadfail/MonadFailWarnings.hs b/testsuite/tests/monadfail/MonadFailWarnings.hs
deleted file mode 100644
index 7e3d7fc428..0000000000
--- a/testsuite/tests/monadfail/MonadFailWarnings.hs
+++ /dev/null
@@ -1,107 +0,0 @@
--- Test purpose:
--- Ensure that MonadFail warnings are issued correctly if the warning flag
--- is enabled
-{-# LANGUAGE NoMonadFailDesugaring #-}
-{-# OPTIONS_GHC -Wmissing-monadfail-instances -Wno-error=compat #-}
-
-module MonadFailWarnings where
-
-import Control.Monad.Fail
-import Control.Monad.ST
-import Data.Functor.Identity
-
-
-
--- should warn, because the do-block gets a general Monad constraint,
--- but should have MonadFail
-general :: Monad m => m a
-general = do
-    Just x <- undefined
-    undefined
-
-
-
--- should NOT warn, because the constraint is correct
-general' :: MonadFail m => m a
-general' = do
-    Just x <- undefined
-    undefined
-
-
-
--- should warn, because Identity isn't MonadFail
-identity :: Identity a
-identity = do
-    Just x <- undefined
-    undefined
-
-
-
--- should NOT warn, because IO is MonadFail
-io :: IO a
-io = do
-    Just x <- undefined
-    undefined
-
-
-
--- should warn, because (ST s) is not MonadFail
-st :: ST s a
-st = do
-    Just x <- undefined
-    undefined
-
-
-
--- should warn, because (r ->) is not MonadFail
-reader :: r -> a
-reader = do
-    Just x <- undefined
-    undefined
-
-
-
--- should NOT warn, because matching against newtype
-newtype Newtype a = Newtype a
-newtypeMatch :: Identity a
-newtypeMatch = do
-    Newtype x <- undefined
-    undefined
-
-
-
--- should NOT warn, because Data has only one constructor
-data Data a = Data a
-singleConMatch :: Identity a
-singleConMatch = do
-    Data x <- undefined
-    undefined
-
-
-
--- should NOT warn, because Maybe' has a MonadFail instance
-data Maybe' a = Nothing' | Just' a
-instance Functor Maybe' where fmap = undefined
-instance Applicative Maybe' where pure = undefined; (<*>) = undefined
-instance Monad Maybe' where (>>=) = undefined
-instance MonadFail Maybe' where fail = undefined
-customFailable :: Maybe' a
-customFailable = do
-    Just x <- undefined
-    undefined
-
-
--- should NOT warn, because patterns always match
-wildcardx, explicitlyIrrefutable, wildcard_, tuple :: Monad m => m a
-wildcardx = do
-    x <- undefined
-    undefined
-explicitlyIrrefutable = do
-    ~(x:y) <- undefined
-    undefined
-wildcard_ = do
-    _ <- undefined
-    undefined
-tuple = do
-    (a,b) <- undefined
-    undefined
diff --git a/testsuite/tests/monadfail/MonadFailWarnings.stderr b/testsuite/tests/monadfail/MonadFailWarnings.stderr
deleted file mode 100644
index 7a341fc816..0000000000
--- a/testsuite/tests/monadfail/MonadFailWarnings.stderr
+++ /dev/null
@@ -1,47 +0,0 @@
-
-MonadFailWarnings.hs:19:5: warning: [-Wmissing-monadfail-instances (in -Wcompat)]
-    • Could not deduce (MonadFail m)
-        arising from the failable pattern ‘Just x’
-        (this will become an error in a future GHC release)
-      from the context: Monad m
-        bound by the type signature for:
-                   general :: forall (m :: * -> *) a. Monad m => m a
-        at MonadFailWarnings.hs:17:1-25
-      Possible fix:
-        add (MonadFail m) to the context of
-          the type signature for:
-            general :: forall (m :: * -> *) a. Monad m => m a
-    • In a stmt of a 'do' block: Just x <- undefined
-      In the expression:
-        do Just x <- undefined
-           undefined
-      In an equation for ‘general’:
-          general
-            = do Just x <- undefined
-                 undefined
-
-MonadFailWarnings.hs:35:5: warning: [-Wmissing-monadfail-instances (in -Wcompat)]
-    • No instance for (MonadFail Identity)
-        arising from the failable pattern ‘Just x’
-        (this will become an error in a future GHC release)
-    • In a stmt of a 'do' block: Just x <- undefined
-      In the expression:
-        do Just x <- undefined
-           undefined
-      In an equation for ‘identity’:
-          identity
-            = do Just x <- undefined
-                 undefined
-
-MonadFailWarnings.hs:59:5: warning: [-Wmissing-monadfail-instances (in -Wcompat)]
-    • No instance for (MonadFail ((->) r))
-        arising from the failable pattern ‘Just x’
-        (this will become an error in a future GHC release)
-    • In a stmt of a 'do' block: Just x <- undefined
-      In the expression:
-        do Just x <- undefined
-           undefined
-      In an equation for ‘reader’:
-          reader
-            = do Just x <- undefined
-                 undefined
diff --git a/testsuite/tests/monadfail/MonadFailWarningsDisabled.hs b/testsuite/tests/monadfail/MonadFailWarningsDisabled.hs
deleted file mode 100644
index 858a212b45..0000000000
--- a/testsuite/tests/monadfail/MonadFailWarningsDisabled.hs
+++ /dev/null
@@ -1,100 +0,0 @@
--- Test purpose:
--- Make sure that not enabling MonadFail warnings makes code compile just
--- as it did in < 8.0
-
--- NOTE: starting w/ GHC 8.6 sugaring is turned on by default; so we have
---       to disable to keep supporting this test-case
---
-{-# LANGUAGE NoMonadFailDesugaring #-}
-{-# OPTIONS -Wno-missing-monadfail-instances #-}
-
-module MonadFailWarnings where
-
-import Control.Monad.Fail
-import Control.Monad.ST
-import Data.Functor.Identity
-
-
-
-general :: Monad m => m a
-general = do
-    Just x <- undefined
-    undefined
-
-
-
-general' :: MonadFail m => m a
-general' = do
-    Just x <- undefined
-    undefined
-
-
-
-identity :: Identity a
-identity = do
-    Just x <- undefined
-    undefined
-
-
-
-io :: IO a
-io = do
-    Just x <- undefined
-    undefined
-
-
-
-st :: ST s a
-st = do
-    Just x <- undefined
-    undefined
-
-
-
-reader :: r -> a
-reader = do
-    Just x <- undefined
-    undefined
-
-
-
-newtype Newtype a = Newtype a
-newtypeMatch :: Identity a
-newtypeMatch = do
-    Newtype x <- undefined
-    undefined
-
-
-
-data Data a = Data a
-singleConMatch :: Identity a
-singleConMatch = do
-    Data x <- undefined
-    undefined
-
-
-
-data Maybe' a = Nothing' | Just' a
-instance Functor Maybe' where fmap = undefined
-instance Applicative Maybe' where pure = undefined; (<*>) = undefined
-instance Monad Maybe' where (>>=) = undefined
-instance MonadFail Maybe' where fail = undefined
-customFailable :: Maybe' a
-customFailable = do
-    Just x <- undefined
-    undefined
-
-
-wildcardx, explicitlyIrrefutable, wildcard_, tuple :: Monad m => m a
-wildcardx = do
-    x <- undefined
-    undefined
-explicitlyIrrefutable = do
-    ~(x:y) <- undefined
-    undefined
-wildcard_ = do
-    _ <- undefined
-    undefined
-tuple = do
-    (a,b) <- undefined
-    undefined
diff --git a/testsuite/tests/monadfail/MonadFailWarningsWithRebindableSyntax.hs b/testsuite/tests/monadfail/MonadFailWarningsWithRebindableSyntax.hs
deleted file mode 100644
index b369fc019f..0000000000
--- a/testsuite/tests/monadfail/MonadFailWarningsWithRebindableSyntax.hs
+++ /dev/null
@@ -1,14 +0,0 @@
--- Test purpose:
--- RebindableSyntax does not play that well with MonadFail, so here we ensure
--- that when both settings are enabled we get the proper warning.
-
-{-# OPTIONS_GHC -Wmissing-monadfail-instances -Wno-error=compat #-}
-{-# LANGUAGE RebindableSyntax #-}
-
-module MonadFailWarningsWithRebindableSyntax where
-
-import Prelude
-
-test1 f g = do
-    Just x <- f
-    g
diff --git a/testsuite/tests/monadfail/MonadFailWarningsWithRebindableSyntax.stderr b/testsuite/tests/monadfail/MonadFailWarningsWithRebindableSyntax.stderr
deleted file mode 100644
index 7b6cd1ba6d..0000000000
--- a/testsuite/tests/monadfail/MonadFailWarningsWithRebindableSyntax.stderr
+++ /dev/null
@@ -1,5 +0,0 @@
-
-MonadFailWarningsWithRebindableSyntax.hs:13:5: warning: [-Wmissing-monadfail-instances (in -Wcompat)]
-    The failable pattern ‘Just x’
-      is used together with -XRebindableSyntax. If this is intentional,
-      compile with -Wno-missing-monadfail-instances.
diff --git a/testsuite/tests/monadfail/all.T b/testsuite/tests/monadfail/all.T
index 32eddb9e98..8fde0296dc 100644
--- a/testsuite/tests/monadfail/all.T
+++ b/testsuite/tests/monadfail/all.T
@@ -1,4 +1 @@
-test('MonadFailWarnings', normal, compile, [''])
 test('MonadFailErrors', normal, compile_fail, [''])
-test('MonadFailWarningsDisabled', normal, compile, [''])
-test('MonadFailWarningsWithRebindableSyntax', normal, compile, [''])