[project @ 2001-02-21 12:14:45 by simonpj]

More wibbles

diff --git a/ghc/compiler/coreSyn/CoreSat.lhs b/ghc/compiler/coreSyn/CoreSat.lhs
index c993d48..9fdcc09 100644 (file)
--- a/ghc/compiler/coreSyn/CoreSat.lhs
+++ b/ghc/compiler/coreSyn/CoreSat.lhs
@@ -386,10 +386,23 @@ mkNonRec :: Id  -> RhsDemand                      -- Lhs: id with demand
         -> OrdList FloatingBind -> CoreExpr    -- Rhs: let binds in body
         -> UniqSM (OrdList FloatingBind)
 mkNonRec bndr dem floats rhs
-  | exprIsValue rhs            -- Notably constructor applications
-  = ASSERT( allLazy floats )   -- The only floats we can get out of a value are eta expansions 
-                               -- e.g.  C $wJust ==> let s = \x -> $wJust x in C s
-                               -- Here we want to float the s binding.
+  | exprIsValue rhs && allLazy floats          -- Notably constructor applications
+  =    -- Why the test for allLazy? You might think that the only 
+       -- floats we can get out of a value are eta expansions 
+       -- e.g.  C $wJust ==> let s = \x -> $wJust x in C s
+       -- Here we want to float the s binding.
+       --
+       -- But if the programmer writes this:
+       --      f x = case x of { (a,b) -> \y -> a }
+       -- then the strictness analyser may say that f has strictness "S"
+       -- Later the eta expander will transform to
+       --      f x y = case x of { (a,b) -> a }
+       -- So now f has arity 2.  Now CoreSat may see
+       --      v = f E
+       -- so the E argument will turn into a FloatCase.  
+       -- Indeed we should end up with
+       --      v = case E of { r -> f r }
+       -- That is, we should not float, even though (f r) is a value
     returnUs (floats `snocOL` FloatLet (NonRec bndr rhs))
     
   |  isUnLiftedType bndr_rep_ty        || isStrictDem dem