Implement BlockArguments (#10843)

This patch implements the BlockArguments extension, as proposed at
https://github.com/ghc-proposals/ghc-proposals/pull/90. It also
fixes #10855 as a side-effect.

This patch adds a large number of shift-reduce conflicts to the parser.
All of them concern the ambiguity as to where constructs like `if` and
`let` end. Fortunately they are resolved correctly by preferring shift.

The patch is based on @gibiansky's ArgumentDo implementation (D1219).

Test Plan: ./validate

Reviewers: goldfire, bgamari, alanz, mpickering

Reviewed By: bgamari, mpickering

Subscribers: Wizek, dfeuer, gibiansky, rwbarton, thomie, mpickering, carter

GHC Trac Issues: #10843, #10855

Differential Revision: https://phabricator.haskell.org/D4260

1 files changed, 8 insertions, 9 deletions

diff --git a/compiler/parser/Lexer.x b/compiler/parser/Lexer.x
index 2f5eccd7e6..83beef210d 100644
--- a/compiler/parser/Lexer.x
+++ b/compiler/parser/Lexer.x
@@ -1244,15 +1244,14 @@ varid :: Action
 varid span buf len =
   case lookupUFM reservedWordsFM fs of
     Just (ITcase, _) -> do
-      lambdaCase <- extension lambdaCaseEnabled
-      keyword <- if lambdaCase
-                 then do
-                   lastTk <- getLastTk
-                   return $ case lastTk of
-                     Just ITlam -> ITlcase
-                     _          -> ITcase
-                 else
-                   return ITcase
+      lastTk <- getLastTk
+      keyword <- case lastTk of
+        Just ITlam -> do
+          lambdaCase <- extension lambdaCaseEnabled
+          if lambdaCase
+            then return ITlcase
+            else failMsgP "Illegal lambda-case (use -XLambdaCase)"
+        _ -> return ITcase
       maybe_layout keyword
       return $ L span keyword
     Just (ITstatic, _) -> do