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, 6 insertions, 3 deletions

diff --git a/ghc/GHCi/UI.hs b/ghc/GHCi/UI.hs
index bcb6d6e38c..1f862de4cb 100644
--- a/ghc/GHCi/UI.hs
+++ b/ghc/GHCi/UI.hs
@@ -889,7 +889,10 @@ installInteractivePrint :: Maybe String -> Bool -> GHCi ()
 installInteractivePrint Nothing _  = return ()
 installInteractivePrint (Just ipFun) exprmode = do
   ok <- trySuccess $ do
-                (name:_) <- GHC.parseName ipFun
+                names <- GHC.parseName ipFun
+                let name = case names of
+                             name':_ -> name'
+                             [] -> panic "installInteractivePrint"
                 modifySession (\he -> let new_ic = setInteractivePrintName (hsc_IC he) name
                                       in he{hsc_IC = new_ic})
                 return Succeeded
@@ -3249,7 +3252,7 @@ stepLocalCmd arg = withSandboxOnly ":steplocal" $ step arg
       case mb_span of
         Nothing  -> stepCmd []
         Just loc -> do
-           Just md <- getCurrentBreakModule
+           md <- fromMaybe (panic "stepLocalCmd") <$> getCurrentBreakModule
            current_toplevel_decl <- enclosingTickSpan md loc
            doContinue (`isSubspanOf` RealSrcSpan current_toplevel_decl) GHC.SingleStep
 
@@ -3740,7 +3743,7 @@ turnOffBreak loc = do
 
 getModBreak :: Module -> GHCi (ForeignRef BreakArray, Array Int SrcSpan)
 getModBreak m = do
-   Just mod_info <- GHC.getModuleInfo m
+   mod_info      <- fromMaybe (panic "getModBreak") <$> GHC.getModuleInfo m
    let modBreaks  = GHC.modInfoModBreaks mod_info
    let arr        = GHC.modBreaks_flags modBreaks
    let ticks      = GHC.modBreaks_locs  modBreaks