X-Git-Url: http://git.megacz.com/?a=blobdiff_plain;f=compiler%2FbasicTypes%2FMkId.lhs;h=7060c803060a0f74bdad35849aa040cbe8dbe9fa;hb=291f2adda098f7e275db3720ed07e7dc77c821c4;hp=a85b69c682c50ce4a71c8a247919b33189e1748e;hpb=0abcc75505992b925ca1b6fed6c97cb105b6fe96;p=ghc-hetmet.git

diff --git a/compiler/basicTypes/MkId.lhs b/compiler/basicTypes/MkId.lhs
index a85b69c..7060c80 100644
--- a/compiler/basicTypes/MkId.lhs
+++ b/compiler/basicTypes/MkId.lhs
@@ -50,7 +50,6 @@ import TysPrim
 import TysWiredIn
 import PrelRules
 import Type
-import TypeRep
 import Coercion
 import TcType
 import CoreUtils	( exprType, mkCoerce )
@@ -60,7 +59,6 @@ import TyCon
 import Class
 import VarSet
 import Name
-import OccName
 import PrimOp
 import ForeignCall
 import DataCon
@@ -70,7 +68,6 @@ import IdInfo
 import NewDemand
 import CoreSyn
 import Unique
-import Maybes
 import PrelNames
 import BasicTypes       hiding ( SuccessFlag(..) )
 import Util
@@ -904,11 +901,7 @@ nullAddrId = pcMiscPrelId nullAddrName addrPrimTy info
            mkCompulsoryUnfolding (Lit nullAddrLit)
 
 ------------------------------------------------
-seqId :: Id
--- 'seq' is very special.  See notes with
---	See DsUtils.lhs Note [Desugaring seq (1)] and
---			Note [Desugaring seq (2)] and
--- Fixity is set in LoadIface.ghcPrimIface
+seqId :: Id	-- See Note [seqId magic]
 seqId = pcMiscPrelId seqName ty info
   where
     info = noCafIdInfo `setUnfoldingInfo` mkCompulsoryUnfolding rhs
@@ -927,6 +920,44 @@ lazyId = pcMiscPrelId lazyIdName ty info
     ty  = mkForAllTys [alphaTyVar] (mkFunTy alphaTy alphaTy)
 \end{code}
 
+Note [seqId magic]
+~~~~~~~~~~~~~~~~~~
+'GHC.Prim.seq' is special in several ways.  
+
+a) Its second arg can have an unboxed type
+      x `seq` (v +# w)
+
+b) Its fixity is set in LoadIface.ghcPrimIface
+
+c) It has quite a bit of desugaring magic. 
+   See DsUtils.lhs Note [Desugaring seq (1)] and (2) and (3)
+
+d) There is some special rule handing: Note [RULES for seq]
+
+Note [Rules for seq]
+~~~~~~~~~~~~~~~~~~~~
+Roman found situations where he had
+      case (f n) of _ -> e
+where he knew that f (which was strict in n) would terminate if n did.
+Notice that the result of (f n) is discarded. So it makes sense to
+transform to
+      case n of _ -> e
+
+Rather than attempt some general analysis to support this, I've added
+enough support that you can do this using a rewrite rule:
+
+  RULE "f/seq" forall n.  seq (f n) e = seq n e
+
+You write that rule.  When GHC sees a case expression that discards
+its result, it mentally transforms it to a call to 'seq' and looks for
+a RULE.  (This is done in Simplify.rebuildCase.)  As usual, the
+correctness of the rule is up to you.
+
+To make this work, we need to be careful that the magical desugaring
+done in Note [seqId magic] item (c) is *not* done on the LHS of a rule.
+Or rather, we arrange to un-do it, in DsBinds.decomposeRuleLhs.
+
+
 Note [lazyId magic]
 ~~~~~~~~~~~~~~~~~~~
     lazy :: forall a?. a? -> a?   (i.e. works for unboxed types too)