Improve MonadFail errors by mentioning the pattern

Example error message:

    MonadFailErrors.hs:16:5: error:
        Could not deduce (MonadFail m)
          arising from a do statement
                       with the failable pattern ‘Just x’

Depends on D1248

Reviewers: austin, bgamari

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D1489

1 files changed, 18 insertions, 6 deletions

diff --git a/testsuite/tests/monadfail/MonadFailErrors.stderr b/testsuite/tests/monadfail/MonadFailErrors.stderr
index ad661772c7..17807a4be0 100644
--- a/testsuite/tests/monadfail/MonadFailErrors.stderr
+++ b/testsuite/tests/monadfail/MonadFailErrors.stderr
@@ -1,6 +1,8 @@
 
 MonadFailErrors.hs:16:5: error:
-    Could not deduce (MonadFail m) arising from a do statement
+    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
@@ -19,7 +21,9 @@ MonadFailErrors.hs:16:5: error:
                  undefined }
 
 MonadFailErrors.hs:30:5: error:
-    No instance for (MonadFail Identity) arising from a do statement
+    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;
@@ -30,7 +34,9 @@ MonadFailErrors.hs:30:5: error:
                  undefined }
 
 MonadFailErrors.hs:44:5: error:
-    No instance for (MonadFail (ST s)) arising from a do statement
+    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;
@@ -41,7 +47,9 @@ MonadFailErrors.hs:44:5: error:
                  undefined }
 
 MonadFailErrors.hs:51:5: error:
-    No instance for (MonadFail ((->) r)) arising from a do statement
+    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;
@@ -52,7 +60,9 @@ MonadFailErrors.hs:51:5: error:
                  undefined }
 
 MonadFailErrors.hs:59:5: error:
-    No instance for (MonadFail Identity) arising from a do statement
+    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;
@@ -63,7 +73,9 @@ MonadFailErrors.hs:59:5: error:
                  undefined }
 
 MonadFailErrors.hs:67:5: error:
-    No instance for (MonadFail Identity) arising from a do statement
+    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;