Expression/command ambiguity resolution

This patch removes 'HsArrApp' and 'HsArrForm' from 'HsExpr' by
introducing a new ambiguity resolution system in the parser.

Problem: there are places in the grammar where we do not know whether we
are parsing an expression or a command:

	proc x -> do { (stuff) -< x }   -- 'stuff' is an expression
	proc x -> do { (stuff) }        -- 'stuff' is a command

Until we encounter arrow syntax (-<) we don't know whether to parse
'stuff' as an expression or a command.

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

	checkCommand :: LHsExpr GhcPs -> P (LHsCmd GhcPs)

This meant polluting 'HsExpr' with command-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 by panicking.

We fix this abstraction leak by parsing into an intermediate
representation, 'ExpCmd':

	data ExpCmdG b where
	  ExpG :: ExpCmdG HsExpr
	  CmdG :: ExpCmdG HsCmd

	type ExpCmd = forall b. ExpCmdG b -> PV (Located (b GhcPs))

	checkExp :: ExpCmd -> PV (LHsExpr GhcPs)
	checkCmd :: ExpCmd -> PV (LHsCmd GhcPs)
	checkExp f = f ExpG  -- interpret as an expression
	checkCmd f = f CmdG  -- interpret as a command

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/command ambiguity.

Future work: apply the same principles to the expression/pattern
ambiguity.

1 files changed, 0 insertions, 12 deletions

diff --git a/compiler/rename/RnExpr.hs b/compiler/rename/RnExpr.hs
index c74e46df97..bed53ece35 100644
--- a/compiler/rename/RnExpr.hs
+++ b/compiler/rename/RnExpr.hs
@@ -412,24 +412,12 @@ rnExpr (HsProc x pat body)
       { (body',fvBody) <- rnCmdTop body
       ; return (HsProc x pat' body', fvBody) }
 
--- Ideally, these would be done in parsing, but to keep parsing simple, we do it here.
-rnExpr e@(HsArrApp {})  = arrowFail e
-rnExpr e@(HsArrForm {}) = arrowFail e
-
 rnExpr other = pprPanic "rnExpr: unexpected expression" (ppr other)
         -- HsWrap
 
 hsHoleExpr :: HsExpr (GhcPass id)
 hsHoleExpr = HsUnboundVar noExt (TrueExprHole (mkVarOcc "_"))
 
-arrowFail :: HsExpr GhcPs -> RnM (HsExpr GhcRn, FreeVars)
-arrowFail e
-  = do { addErr (vcat [ text "Arrow command found where an expression was expected:"
-                      , nest 2 (ppr e) ])
-         -- Return a place-holder hole, so that we can carry on
-         -- to report other errors
-       ; return (hsHoleExpr, emptyFVs) }
-
 ----------------------
 -- See Note [Parsing sections] in Parser.y
 rnSection :: HsExpr GhcPs -> RnM (HsExpr GhcRn, FreeVars)