Make GHCi work when RebindableSyntax is enabled

Previously, we were running some blocks of code at the start of every
GHCi sessions which use do-notation, something which doesn't work well
if you start GHCi with the `-XRebindableSyntax` flag on. This tweaks the
code to avoid the use of do-notation so that `-XRebindableSyntax` won't
reject it.

Test Plan: make test TEST=T13385

Reviewers: austin, bgamari

Reviewed By: bgamari

Subscribers: rwbarton, thomie

GHC Trac Issues: #13385

Differential Revision: https://phabricator.haskell.org/D3621

1 files changed, 7 insertions, 5 deletions

diff --git a/ghc/GHCi/UI/Monad.hs b/ghc/GHCi/UI/Monad.hs
index 244595b8b0..b57a5a0a64 100644
--- a/ghc/GHCi/UI/Monad.hs
+++ b/ghc/GHCi/UI/Monad.hs
@@ -420,13 +420,15 @@ foreign import ccall "revertCAFs" rts_revertCAFs  :: IO ()
 -- | Compile "hFlush stdout; hFlush stderr" once, so we can use it repeatedly
 initInterpBuffering :: Ghc (ForeignHValue, ForeignHValue)
 initInterpBuffering = do
+  -- We take great care not to use do-notation in the expressions below, as
+  -- they are fragile in the presence of RebindableSyntax (Trac #13385).
   nobuf <- GHC.compileExprRemote $
-   "do { System.IO.hSetBuffering System.IO.stdin System.IO.NoBuffering; " ++
-       " System.IO.hSetBuffering System.IO.stdout System.IO.NoBuffering; " ++
-       " System.IO.hSetBuffering System.IO.stderr System.IO.NoBuffering }"
+   "                  System.IO.hSetBuffering System.IO.stdin  System.IO.NoBuffering" ++
+   "`GHC.Base.thenIO` System.IO.hSetBuffering System.IO.stdout System.IO.NoBuffering" ++
+   "`GHC.Base.thenIO` System.IO.hSetBuffering System.IO.stderr System.IO.NoBuffering"
   flush <- GHC.compileExprRemote $
-   "do { System.IO.hFlush System.IO.stdout; " ++
-       " System.IO.hFlush System.IO.stderr }"
+   "                  System.IO.hFlush System.IO.stdout" ++
+   "`GHC.Base.thenIO` System.IO.hFlush System.IO.stderr"
   return (nobuf, flush)
 
 -- | Invoke "hFlush stdout; hFlush stderr" in the interpreter