Pattern/expression ambiguity resolution

This patch removes 'EWildPat', 'EAsPat', 'EViewPat', and 'ELazyPat'
from 'HsExpr' by using the ambiguity resolution system introduced
earlier for the command/expression ambiguity.

Problem: there are places in the grammar where we do not know whether we
are parsing an expression or a pattern, for example:

	do { Con a b <- x } -- 'Con a b' is a pattern
	do { Con a b }      -- 'Con a b' is an expression

Until we encounter binding syntax (<-) we don't know whether to parse
'Con a b' as an expression or a pattern.

The old solution was to parse as HsExpr always, and rejig later:

	checkPattern :: LHsExpr GhcPs -> P (LPat GhcPs)

This meant polluting 'HsExpr' with pattern-related constructors. In
other words, limitations of the parser were affecting the AST, and all
other code (the renamer, the typechecker) had to deal with these extra
constructors.

We fix this abstraction leak by parsing into an overloaded
representation:

	class DisambECP b where ...
	newtype ECP = ECP { runECP_PV :: forall b. DisambECP b => PV (Located b) }

See Note [Ambiguous syntactic categories] for details.

Now the intricacies of parsing have no effect on the hsSyn AST when it
comes to the expression/pattern ambiguity.

1 files changed, 3 insertions, 27 deletions

diff --git a/compiler/rename/RnExpr.hs b/compiler/rename/RnExpr.hs
index dd38feb367..7b00a62403 100644
--- a/compiler/rename/RnExpr.hs
+++ b/compiler/rename/RnExpr.hs
@@ -140,6 +140,9 @@ rnExpr (HsVar _ (L l v))
 rnExpr (HsIPVar x v)
   = return (HsIPVar x v, emptyFVs)
 
+rnExpr (HsUnboundVar x v)
+  = return (HsUnboundVar x v, emptyFVs)
+
 rnExpr (HsOverLabel x _ v)
   = do { rebindable_on <- xoptM LangExt.RebindableSyntax
        ; if rebindable_on
@@ -346,24 +349,6 @@ rnExpr (ArithSeq x _ seq)
             return (ArithSeq x Nothing new_seq, fvs) }
 
 {-
-These three are pattern syntax appearing in expressions.
-Since all the symbols are reservedops we can simply reject them.
-We return a (bogus) EWildPat in each case.
--}
-
-rnExpr (EWildPat _)  = return (hsHoleExpr, emptyFVs)   -- "_" is just a hole
-rnExpr e@(EAsPat {})
-  = do { opt_TypeApplications <- xoptM LangExt.TypeApplications
-       ; let msg | opt_TypeApplications
-                    = "Type application syntax requires a space before '@'"
-                 | otherwise
-                    = "Did you mean to enable TypeApplications?"
-       ; patSynErr e (text msg)
-       }
-rnExpr e@(EViewPat {}) = patSynErr e empty
-rnExpr e@(ELazyPat {}) = patSynErr e empty
-
-{-
 ************************************************************************
 *                                                                      *
         Static values
@@ -415,9 +400,6 @@ rnExpr (HsProc x pat body)
 rnExpr other = pprPanic "rnExpr: unexpected expression" (ppr other)
         -- HsWrap
 
-hsHoleExpr :: HsExpr (GhcPass id)
-hsHoleExpr = HsUnboundVar noExt (TrueExprHole (mkVarOcc "_"))
-
 ----------------------
 -- See Note [Parsing sections] in Parser.y
 rnSection :: HsExpr GhcPs -> RnM (HsExpr GhcRn, FreeVars)
@@ -2087,12 +2069,6 @@ sectionErr expr
   = hang (text "A section must be enclosed in parentheses")
        2 (text "thus:" <+> (parens (ppr expr)))
 
-patSynErr :: HsExpr GhcPs -> SDoc -> RnM (HsExpr GhcRn, FreeVars)
-patSynErr e explanation = do { addErr (sep [text "Pattern syntax in expression context:",
-                                nest 4 (ppr e)] $$
-                                  explanation)
-                 ; return (EWildPat noExt, emptyFVs) }
-
 badIpBinds :: Outputable a => SDoc -> a -> SDoc
 badIpBinds what binds
   = hang (text "Implicit-parameter bindings illegal in" <+> what)