Simplify the MonadFail code

Simplify and tidy up the MonadFail code.
See TcMatches.tcMonadFailOp

Less, code; and more robust.

This incidentally fixes a bug; see the change
to MonadFailErrors.stderr

1 files changed, 52 insertions, 78 deletions

diff --git a/testsuite/tests/monadfail/MonadFailErrors.stderr b/testsuite/tests/monadfail/MonadFailErrors.stderr
index 17807a4be0..8a478eecf3 100644
--- a/testsuite/tests/monadfail/MonadFailErrors.stderr
+++ b/testsuite/tests/monadfail/MonadFailErrors.stderr
@@ -1,86 +1,60 @@
 
 MonadFailErrors.hs:16:5: 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 :: Monad m => m a
-      at MonadFailErrors.hs:14:12-25
-    Possible fix:
-      add (MonadFail m) to the context of
-        the type signature for:
-          general :: 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 }
+    • 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 :: Monad m => m a
+        at MonadFailErrors.hs:14:12-25
+      Possible fix:
+        add (MonadFail m) to the context of
+          the type signature for:
+            general :: 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 }
 
 MonadFailErrors.hs:30:5: error:
-    No instance for (MonadFail Identity)
-      arising from a do statement
-                   with the failable pattern ‘Just x’
-    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 }
+    • No instance for (MonadFail Identity)
+        arising from a do statement
+        with the failable pattern ‘Just x’
+    • 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 }
 
 MonadFailErrors.hs:44:5: error:
-    No instance for (MonadFail (ST s))
-      arising from a do statement
-                   with the failable pattern ‘Just x’
-    In a stmt of a 'do' block: Just x <- undefined
-    In the expression:
-      do { Just x <- undefined;
-           undefined }
-    In an equation for ‘st’:
-        st
-          = do { Just x <- undefined;
-                 undefined }
+    • No instance for (MonadFail (ST s))
+        arising from a do statement
+        with the failable pattern ‘Just x’
+    • In a stmt of a 'do' block: Just x <- undefined
+      In the expression:
+        do { Just x <- undefined;
+             undefined }
+      In an equation for ‘st’:
+          st
+            = do { Just x <- undefined;
+                   undefined }
 
 MonadFailErrors.hs:51:5: error:
-    No instance for (MonadFail ((->) r))
-      arising from a do statement
-                   with the failable pattern ‘Just x’
-    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 }
-
-MonadFailErrors.hs:59:5: error:
-    No instance for (MonadFail Identity)
-      arising from a do statement
-                   with the failable pattern ‘Newtype x’
-    In a stmt of a 'do' block: Newtype x <- undefined
-    In the expression:
-      do { Newtype x <- undefined;
-           undefined }
-    In an equation for ‘newtypeMatch’:
-        newtypeMatch
-          = do { Newtype x <- undefined;
-                 undefined }
-
-MonadFailErrors.hs:67:5: error:
-    No instance for (MonadFail Identity)
-      arising from a do statement
-                   with the failable pattern ‘Data x’
-    In a stmt of a 'do' block: Data x <- undefined
-    In the expression:
-      do { Data x <- undefined;
-           undefined }
-    In an equation for ‘singleConMatch’:
-        singleConMatch
-          = do { Data x <- undefined;
-                 undefined }
+    • No instance for (MonadFail ((->) r))
+        arising from a do statement
+        with the failable pattern ‘Just x’
+    • 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 }