From f7df28a48a9d2901d9e27e0e9970b3f692bc9b3f Mon Sep 17 00:00:00 2001
From: "simonpj@microsoft.com" <unknown>
Date: Fri, 21 Aug 2009 09:06:37 +0000
Subject: [PATCH] Wibbles to field-label puns

---
 compiler/hsSyn/HsPat.lhs     |   29 ++++++++++++++++++++---------
 compiler/parser/Parser.y.pp  |    2 +-
 compiler/parser/RdrHsSyn.lhs |    9 ++++++++-
 3 files changed, 29 insertions(+), 11 deletions(-)

diff --git a/compiler/hsSyn/HsPat.lhs b/compiler/hsSyn/HsPat.lhs
index 84eadb7..c886766 100644
--- a/compiler/hsSyn/HsPat.lhs
+++ b/compiler/hsSyn/HsPat.lhs
@@ -177,13 +177,21 @@ data HsRecFields id arg 	-- A bunch of record fields
 				--	{ x = 3, y = True }
 	-- Used for both expressions and patterns
   = HsRecFields { rec_flds   :: [HsRecField id arg],
-		  rec_dotdot :: Maybe Int }
-	-- Nothing => the normal case
-	-- Just n  => the group uses ".." notation, 
-	--		and the first n elts of rec_flds
-	--		were the user-written ones
-	-- (In the latter case, the remaining elts of
-	--  rec_flds are the non-user-written ones)
+		  rec_dotdot :: Maybe Int }  -- Note [DotDot fields]
+
+-- Note [DotDot fields]
+-- ~~~~~~~~~~~~~~~~~~~~
+-- The rec_dotdot field means this:
+--   Nothing => the normal case
+--   Just n  => the group uses ".." notation, 
+--
+-- In the latter case: 
+--
+--   *before* renamer: rec_flds are exactly the n user-written fields
+--
+--   *after* renamer:  rec_flds includes *all* fields, with 
+--   	     	       the first 'n' being the user-written ones
+--		       and the remainder being 'filled in' implicitly
 
 data HsRecField id arg = HsRecField {
 	hsRecFieldId  :: Located id,
@@ -196,9 +204,12 @@ data HsRecField id arg = HsRecField {
 -- If you write T { x, y = v+1 }, the HsRecFields will be
 --	HsRecField x x True ...
 --	HsRecField y (v+1) False ...
--- That is, for "punned" field x is immediately expanded to x=x
--- but with a punning flag so we can detect it later
+-- That is, for "punned" field x is expanded (in the renamer) 
+-- to x=x; but with a punning flag so we can detect it later
 -- (e.g. when pretty printing)
+--
+-- If the original field was qualified, we un-qualify it, thus
+--    T { A.x } means T { A.x = x }
 
 hsRecFields :: HsRecFields id arg -> [id]
 hsRecFields rbinds = map (unLoc . hsRecFieldId) (rec_flds rbinds)
diff --git a/compiler/parser/Parser.y.pp b/compiler/parser/Parser.y.pp
index c2b6aee..a9b96a0 100644
--- a/compiler/parser/Parser.y.pp
+++ b/compiler/parser/Parser.y.pp
@@ -1639,7 +1639,7 @@ fbinds1	:: { ([HsRecField RdrName (LHsExpr RdrName)], Bool) }
   
 fbind	:: { HsRecField RdrName (LHsExpr RdrName) }
 	: qvar '=' exp	{ HsRecField $1 $3 False }
-	| qvar 		{ HsRecField $1 (L (getLoc $1) (HsVar (unLoc $1))) True }
+        | qvar          { HsRecField $1 (L (getLoc $1) placeHolderPunRhs) True }
 			-- Here's where we say that plain 'x'
 			-- means exactly 'x = x'.  The pun-flag boolean is
 			-- there so we can still print it right
diff --git a/compiler/parser/RdrHsSyn.lhs b/compiler/parser/RdrHsSyn.lhs
index 59dfe02..e8d00a2 100644
--- a/compiler/parser/RdrHsSyn.lhs
+++ b/compiler/parser/RdrHsSyn.lhs
@@ -19,6 +19,7 @@ module RdrHsSyn (
         cvBindsAndSigs,
 	cvTopDecls,
 	findSplice, checkDecBrGroup,
+        placeHolderPunRhs,
 
 	-- Stuff to do with Foreign declarations
 	mkImport,
@@ -789,9 +790,15 @@ checkAPat dynflags loc e = case e of
    HsType ty          -> return (TypePat ty) 
    _                  -> patFail loc
 
-plus_RDR, bang_RDR :: RdrName
+placeHolderPunRhs :: HsExpr RdrName
+-- The RHS of a punned record field will be filled in by the renamer
+-- It's better not to make it an error, in case we want to print it when debugging
+placeHolderPunRhs = HsVar pun_RDR
+
+plus_RDR, bang_RDR, pun_RDR :: RdrName
 plus_RDR = mkUnqual varName (fsLit "+")	-- Hack
 bang_RDR = mkUnqual varName (fsLit "!")	-- Hack
+pun_RDR  = mkUnqual varName (fsLit "pun-right-hand-side")
 
 checkPatField :: HsRecField RdrName (LHsExpr RdrName) -> P (HsRecField RdrName (LPat RdrName))
 checkPatField fld = do	{ p <- checkLPat (hsRecFieldArg fld)
-- 
1.7.10.4