Turn on MonadFail desugaring by default

Summary:
This contains two commits:

----

Make GHC's code-base compatible w/ `MonadFail`

There were a couple of use-sites which implicitly used pattern-matches
in `do`-notation even though the underlying `Monad` didn't explicitly
support `fail`

This refactoring turns those use-sites into explicit case
discrimations and adds an `MonadFail` instance for `UniqSM`
(`UniqSM` was the worst offender so this has been postponed for a
follow-up refactoring)

---

Turn on MonadFail desugaring by default

This finally implements the phase scheduled for GHC 8.6 according to

https://prime.haskell.org/wiki/Libraries/Proposals/MonadFail#Transitionalstrategy

This also preserves some tests that assumed MonadFail desugaring to be
active; all ghc boot libs were already made compatible with this
`MonadFail` long ago, so no changes were needed there.

Test Plan: Locally performed ./validate --fast

Reviewers: bgamari, simonmar, jrtc27, RyanGlScott

Reviewed By: bgamari

Subscribers: bgamari, RyanGlScott, rwbarton, thomie, carter

Differential Revision: https://phabricator.haskell.org/D5028

1 files changed, 5 insertions, 1 deletions

diff --git a/compiler/main/InteractiveEval.hs b/compiler/main/InteractiveEval.hs
index bec52e6001..452ccb3e80 100644
--- a/compiler/main/InteractiveEval.hs
+++ b/compiler/main/InteractiveEval.hs
@@ -942,7 +942,11 @@ compileParsedExprRemote expr@(L loc _) = withSession $ \hsc_env -> do
         ValBinds noExt
                      (unitBag $ mkHsVarBind loc (getRdrName expr_name) expr) []
 
-  Just ([_id], hvals_io, fix_env) <- liftIO $ hscParsedStmt hsc_env let_stmt
+  pstmt <- liftIO $ hscParsedStmt hsc_env let_stmt
+  let (hvals_io, fix_env) = case pstmt of
+        Just ([_id], hvals_io', fix_env') -> (hvals_io', fix_env')
+        _ -> panic "compileParsedExprRemote"
+
   updateFixityEnv fix_env
   status <- liftIO $ evalStmt hsc_env False (EvalThis hvals_io)
   case status of