[project @ 2001-10-17 11:05:36 by simonpj]

-------------------------------------------
	Desugar bindings into Rec groups more often
	-------------------------------------------

In rather obscure cases (involving functional dependencies)
it is possible to get an AbsBinds [] [] (no tyvars, no dicts)
which nevertheless has some "dictionary bindings".  These
come out of the typechecker in non-dependency order, so we
need to Rec them just in case.

It turns out to be a bit awkward.  The smallest fix is
to make dsLet always make a Rec; brutal but correct.

diff --git a/ghc/compiler/deSugar/DsBinds.lhs b/ghc/compiler/deSugar/DsBinds.lhs
index ddfbd6c..4f2323d 100644 (file)
--- a/ghc/compiler/deSugar/DsBinds.lhs
+++ b/ghc/compiler/deSugar/DsBinds.lhs
@@ -90,12 +90,16 @@ dsMonoBinds auto_scc (PatMonoBind pat grhss locn) rest
     returnDs (sel_binds ++ rest)
 
        -- Common special case: no type or dictionary abstraction
+       -- For the (rare) case when there are some mixed-up
+       -- dictionary bindings (for which a Rec is convenient)
+       -- we reply on the enclosing dsBind to wrap a Rec around.
 dsMonoBinds auto_scc (AbsBinds [] [] exports inlines binds) rest
   = dsMonoBinds (addSccs auto_scc exports) binds []`thenDs` \ core_prs ->
     let 
-       exports' = [(global, Var local) | (_, global, local) <- exports]
+       core_prs' = addLocalInlines exports inlines core_prs
+       exports'  = [(global, Var local) | (_, global, local) <- exports]
     in
-    returnDs (addLocalInlines exports inlines core_prs ++ exports' ++ rest)
+    returnDs (core_prs' ++ exports' ++ rest)
 
        -- Another common case: one exported variable
        -- Non-recursive bindings come through this way
@@ -106,22 +110,26 @@ dsMonoBinds auto_scc
     let 
        -- Always treat the binds as recursive, because the typechecker
        -- makes rather mixed-up dictionary bindings
-       core_binds = [Rec core_prs]
-       global'    = (global, mkInline (idName global `elemNameSet` inlines) $
+       core_bind = Rec core_prs
+
+       -- The mkInline does directly what the 
+       -- addLocalInlines do in the other cases
+       export'    = (global, mkInline (idName global `elemNameSet` inlines) $
                              mkLams tyvars $ mkLams dicts $ 
-                             mkDsLets core_binds (Var local))
+                             Let core_bind (Var local))
     in
-    returnDs (global' : rest)
+    returnDs (export' : rest)
 
 dsMonoBinds auto_scc (AbsBinds all_tyvars dicts exports inlines binds) rest
   = dsMonoBinds (addSccs auto_scc exports) binds []`thenDs` \ core_prs ->
     let 
-       core_binds = [Rec (addLocalInlines exports inlines core_prs)]
+       -- Rec because of mixed-up dictionary bindings
+       core_bind = Rec (addLocalInlines exports inlines core_prs)
 
        tup_expr      = mkTupleExpr locals
        tup_ty        = exprType tup_expr
        poly_tup_expr = mkLams all_tyvars $ mkLams dicts $
-                       mkDsLets core_binds tup_expr
+                       Let core_bind tup_expr
        locals        = [local | (_, _, local) <- exports]
        local_tys     = map idType locals
     in

diff --git a/ghc/compiler/deSugar/DsExpr.lhs b/ghc/compiler/deSugar/DsExpr.lhs
index 0693b36..c14935a 100644 (file)
--- a/ghc/compiler/deSugar/DsExpr.lhs
+++ b/ghc/compiler/deSugar/DsExpr.lhs
@@ -33,7 +33,7 @@ import DsBinds                ( dsMonoBinds, AutoScc(..) )
 import DsGRHSs         ( dsGuarded )
 import DsCCall         ( dsCCall, resultWrapper )
 import DsListComp      ( dsListComp )
-import DsUtils         ( mkErrorAppDs, mkDsLets, mkStringLit, mkStringLitFS, 
+import DsUtils         ( mkErrorAppDs, mkStringLit, mkStringLitFS, 
                          mkConsExpr, mkNilExpr, mkIntegerLit
                        )
 import Match           ( matchWrapper, matchSimply )
@@ -106,9 +106,14 @@ dsLet (MonoBind (AbsBinds [] [] binder_triples inlines
 -- Ordinary case for bindings
 dsLet (MonoBind binds sigs is_rec) body
   = dsMonoBinds NoSccs binds []  `thenDs` \ prs ->
-    case is_rec of
-      Recursive    -> returnDs (Let (Rec prs) body)
-      NonRecursive -> returnDs (mkDsLets [NonRec b r | (b,r) <- prs] body)
+    returnDs (Let (Rec prs) body)
+       -- Use a Rec regardless of is_rec. 
+       -- Why? Because it allows the MonoBinds to be all
+       -- mixed up, which is what happens in one rare case
+       -- Namely, for an AbsBind with no tyvars and no dicts,
+       --         but which does have dictionary bindings.
+       -- See notes with TcSimplify.inferLoop [NO TYVARS]
+       -- It turned out that wrapping a Rec here was the easiest solution
 \end{code}
 
 %************************************************************************