Allow RULES for seq, and exploit them

[ghc-hetmet.git] / compiler / deSugar / DsBinds.lhs

diff --git a/compiler/deSugar/DsBinds.lhs b/compiler/deSugar/DsBinds.lhs
index 020b7b4..e65da3c 100644 (file)
--- a/compiler/deSugar/DsBinds.lhs
+++ b/compiler/deSugar/DsBinds.lhs
@@ -36,6 +36,7 @@ import TcType
 import CostCentre
 import Module
 import Id
+import MkId    ( seqId )
 import Var     ( Var, TyVar )
 import VarSet
 import Rules
@@ -163,14 +164,16 @@ dsHsBind auto_scc rest (AbsBinds [] [] exports binds)
   where B is the *non-recursive* binding
        fl = fg a b
        gl = gg b
-       h  = h a b 
+       h  = h a b    -- See (b); note shadowing!
   
   Notice (a) g has a different number of type variables to f, so we must
             use the mkArbitraryType thing to fill in the gaps.  
             We use a type-let to do that.
 
         (b) The local variable h isn't in the exports, and rather than
-            clone a fresh copy we simply replace h by (h a b).  
+            clone a fresh copy we simply replace h by (h a b), where
+            the two h's have different types!  Shadowing happens here,
+            which looks confusing but works fine.
 
         (c) The result is *still* quadratic-sized if there are a lot of
             small bindings.  So if there are more than some small
@@ -474,6 +477,12 @@ decomposeRuleLhs lhs
         -- a LHS:       let f71 = M.f Int in f71
     decomp env (Let (NonRec dict rhs) body) 
         = decomp (extendVarEnv env dict (simpleSubst env rhs)) body
+
+    decomp env (Case scrut bndr ty [(DEFAULT, _, body)])
+        | isDeadBinder bndr    -- Note [Matching seqId]
+        = Just (seqId, [Type (idType bndr), Type ty, 
+                        simpleSubst env scrut, simpleSubst env body])
+
     decomp env body 
         = case collectArgs (simpleSubst env body) of
             (Var fn, args) -> Just (fn, args)
@@ -514,17 +523,23 @@ addInlinePrags prags bndr rhs
        (inl:_) -> addInlineInfo inl bndr rhs
 
 addInlineInfo :: InlineSpec -> Id -> CoreExpr -> (Id,CoreExpr)
-addInlineInfo (Inline phase is_inline) bndr rhs
-  = (attach_phase bndr phase, wrap_inline is_inline rhs)
+addInlineInfo (Inline prag is_inline) bndr rhs
+  = (attach_pragma bndr prag, wrap_inline is_inline rhs)
   where
-    attach_phase bndr phase 
-       | isAlwaysActive phase = bndr   -- Default phase
-       | otherwise            = bndr `setInlinePragma` phase
+    attach_pragma bndr prag
+        | isDefaultInlinePragma prag = bndr
+        | otherwise                  = bndr `setInlinePragma` prag
 
     wrap_inline True  body = mkInlineMe body
     wrap_inline False body = body
 \end{code}
 
+Note [Matching seq]
+~~~~~~~~~~~~~~~~~~~
+The desugarer turns (seq e r) into (case e of _ -> r), via a special-case hack
+and this code turns it back into an application of seq!  
+See Note [Rules for seq] in MkId for the details.
+
 
 %************************************************************************
 %*                                                                     *