doc/bugs/No_MonadFail_instance.mdwn

@@ -0,0 +1,47 @@
+### Please describe the problem.
+
+
+### What steps will reproduce the problem?
+
+Build git-annex with GHC 8.6.
+
+### What version of git-annex are you using? On what operating system?
+
+7.20181211, master
+
+### Please provide any additional information below.
+
+Following the implementation plan for the [MonadFail Proposal](https://wiki.haskell.org/MonadFail_Proposal), GHC 8.6 has enabled `-XMonadFailDesugaring` by default.
+
+Some of the code in git-annex uses failable patterns in `do`-blocks, so it fails to compile starting from GHC 8.6, as there is no `MonadFail` instance for `Annex`. Here is an example of such an error:
+
+    Remote/Glacier.hs:165:17: error:
+        • No instance for (Control.Monad.Fail.MonadFail Annex)
+            arising from a do statement
+            with the failable pattern ‘(_, Just h, _, pid)’
+        • In a stmt of a 'do' block:
+            (_, Just h, _, pid) <- liftIO $ createProcess cmd
+          In the expression:
+            do let cmd = ...
+               (_, Just h, _, pid) <- liftIO $ createProcess cmd
+               ok <- ifM
+                       (liftIO $ hIsEOF h)
+                       (return False, sink =<< liftIO (L.hGetContents h))
+               liftIO $ hClose h
+               ....
+          In an equation for ‘go’:
+              go (Just e)
+                = do let cmd = ...
+                     (_, Just h, _, pid) <- liftIO $ createProcess cmd
+                     ok <- ifM
+                             (liftIO $ hIsEOF h)
+                             (return False, sink =<< liftIO (L.hGetContents h))
+                     ....
+        |
+    165 |                 (_, Just h, _, pid) <- liftIO $ createProcess cmd
+        |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+
+### Have you had any luck using git-annex before? (Sometimes we get tired of reading bug reports all day and a lil' positive end note does wonders)
+
+Just trying to start using it. I have built it with `-XNoMonadFailDesugaring` for now.