Visible type application

This re-working of the typechecker algorithm is based on
the paper "Visible type application", by Richard Eisenberg,
Stephanie Weirich, and Hamidhasan Ahmed, to be published at
ESOP'16.

This patch introduces -XTypeApplications, which allows users
to say, for example `id @Int`, which has type `Int -> Int`. See
the changes to the user manual for details.

This patch addresses tickets #10619, #5296, #10589.

3 files changed, 11 insertions, 9 deletions

diff --git a/compiler/rename/RnExpr.hs b/compiler/rename/RnExpr.hs
index c4e5bb2abe..03f4b62043 100644
--- a/compiler/rename/RnExpr.hs
+++ b/compiler/rename/RnExpr.hs
@@ -298,9 +298,9 @@ rnExpr (HsMultiIf _ty alts)
        -- ; return (HsMultiIf ty alts', fvs) }
        ; return (HsMultiIf placeHolderType alts', fvs) }
 
-rnExpr (HsType a)
-  = do { (t, fvT) <- rnLHsType HsTypeCtx a
-       ; return (HsType t, fvT) }
+rnExpr (HsType ty)
+  = do { (ty', fvT) <- rnHsWcType HsTypeCtx ty
+       ; return (HsType ty', fvT) }
 
 rnExpr (ArithSeq _ _ seq)
   = do { opt_OverloadedLists <- xoptM LangExt.OverloadedLists
@@ -524,7 +524,7 @@ rnCmd (HsCmdDo (L l stmts) _)
             rnStmts ArrowExpr rnLCmd stmts (\ _ -> return ((), emptyFVs))
         ; return ( HsCmdDo (L l stmts') placeHolderType, fvs ) }
 
-rnCmd cmd@(HsCmdCast {}) = pprPanic "rnCmd" (ppr cmd)
+rnCmd cmd@(HsCmdWrap {}) = pprPanic "rnCmd" (ppr cmd)
 
 ---------------------------------------------------
 type CmdNeeds = FreeVars        -- Only inhabitants are
@@ -541,7 +541,7 @@ methodNamesCmd (HsCmdArrApp _arrow _arg _ HsFirstOrderApp _rtl)
 methodNamesCmd (HsCmdArrApp _arrow _arg _ HsHigherOrderApp _rtl)
   = unitFV appAName
 methodNamesCmd (HsCmdArrForm {}) = emptyFVs
-methodNamesCmd (HsCmdCast _ cmd) = methodNamesCmd cmd
+methodNamesCmd (HsCmdWrap _ cmd) = methodNamesCmd cmd
 
 methodNamesCmd (HsCmdPar c) = methodNamesLCmd c
 
@@ -1819,7 +1819,8 @@ sectionErr expr
 
 patSynErr :: HsExpr RdrName -> RnM (HsExpr Name, FreeVars)
 patSynErr e = do { addErr (sep [ptext (sLit "Pattern syntax in expression context:"),
-                                nest 4 (ppr e)])
+                                nest 4 (ppr e)] $$
+                           text "Did you mean to enable TypeApplications?")
                  ; return (EWildPat, emptyFVs) }
 
 badIpBinds :: Outputable a => SDoc -> a -> SDoc
diff --git a/compiler/rename/RnSplice.hs b/compiler/rename/RnSplice.hs
index 8f87d730d8..9ddf132311 100644
--- a/compiler/rename/RnSplice.hs
+++ b/compiler/rename/RnSplice.hs
@@ -45,7 +45,7 @@ import Var              ( Id )
 import THNames          ( quoteExpName, quotePatName, quoteDecName, quoteTypeName
                         , decsQTyConName, expQTyConName, patQTyConName, typeQTyConName, )
 
-import {-# SOURCE #-} TcExpr   ( tcMonoExpr )
+import {-# SOURCE #-} TcExpr   ( tcPolyExpr )
 import {-# SOURCE #-} TcSplice ( runMetaD, runMetaE, runMetaP, runMetaT, tcTopSpliceExpr )
 #endif
 
@@ -295,7 +295,7 @@ runRnSplice flavour run_meta ppr_res splice
              -- Typecheck the expression
        ; meta_exp_ty   <- tcMetaTy meta_ty_name
        ; zonked_q_expr <- tcTopSpliceExpr Untyped $
-                          tcMonoExpr the_expr meta_exp_ty
+                          tcPolyExpr the_expr meta_exp_ty
 
              -- Run the expression
        ; result <- run_meta zonked_q_expr
diff --git a/compiler/rename/RnTypes.hs b/compiler/rename/RnTypes.hs
index 52a164f105..5a58148170 100644
--- a/compiler/rename/RnTypes.hs
+++ b/compiler/rename/RnTypes.hs
@@ -751,7 +751,7 @@ checkExtraConstraintWildCard env wc
   = checkWildCard env mb_bad
   where
     mb_bad | not (extraConstraintWildCardsAllowed env)
-           = Just (ptext (sLit "Extra-contraint wildcard") <+> quotes (ppr wc)
+           = Just (ptext (sLit "Extra-constraint wildcard") <+> quotes (ppr wc)
                    <+> ptext (sLit "not allowed"))
            | otherwise
            = Nothing
@@ -774,6 +774,7 @@ wildCardsAllowed env
        RuleCtx {}          -> True
        FamPatCtx {}        -> True   -- Not named wildcards though
        GHCiCtx {}          -> True
+       HsTypeCtx {}        -> True
        _                   -> False
 
 rnAnonWildCard :: HsWildCardInfo RdrName -> RnM (HsWildCardInfo Name)