checkWellStaged: reverse comparsion (no change in semantics), plus some comments

[ghc-hetmet.git] / compiler / typecheck / TcEnv.lhs

diff --git a/compiler/typecheck/TcEnv.lhs b/compiler/typecheck/TcEnv.lhs
index 4f48f7f..58bda52 100644 (file)
--- a/compiler/typecheck/TcEnv.lhs
+++ b/compiler/typecheck/TcEnv.lhs
@@ -3,6 +3,13 @@
 %
 
 \begin{code}
+{-# OPTIONS -w #-}
+-- The above warning supression flag is a temporary kludge.
+-- While working on this module you are encouraged to remove it and fix
+-- any warnings in the module. See
+--     http://hackage.haskell.org/trac/ghc/wiki/Commentary/CodingStyle#Warnings
+-- for details
+
 module TcEnv(
        TyThing(..), TcTyThing(..), TcId,
 
@@ -546,12 +553,12 @@ thLevel (Brack l _ _) = l
 
 
 checkWellStaged :: SDoc                -- What the stage check is for
-               -> ThLevel      -- Binding level
+               -> ThLevel      -- Binding level (increases inside brackets)
                -> ThStage      -- Use stage
                -> TcM ()       -- Fail if badly staged, adding an error
 checkWellStaged pp_thing bind_lvl use_stage
-  | bind_lvl <= use_lvl        -- OK!
-  = returnM () 
+  | use_lvl >= bind_lvl        -- OK! Used later than bound
+  = returnM ()                 -- E.g.  \x -> [| $(f x) |]
 
   | bind_lvl == topLevel       -- GHC restriction on top level splices
   = failWithTc $ 
@@ -559,7 +566,7 @@ checkWellStaged pp_thing bind_lvl use_stage
         nest 2 (ptext SLIT("is used in a top-level splice, and must be imported, not defined locally"))]
 
   | otherwise                  -- Badly staged
-  = failWithTc $ 
+  = failWithTc $               -- E.g.  \x -> $(f x)
     ptext SLIT("Stage error:") <+> pp_thing <+> 
        hsep   [ptext SLIT("is bound at stage") <+> ppr bind_lvl,
                ptext SLIT("but used at stage") <+> ppr use_lvl]
@@ -628,30 +635,20 @@ iDFunId info = instanceDFunId (iSpec info)
 
 data InstBindings
   = VanillaInst                -- The normal case
-       (LHsBinds Name)         -- Bindings
+       (LHsBinds Name)         -- Bindings for the instance methods
        [LSig Name]             -- User pragmas recorded for generating 
                                -- specialised instances
 
   | NewTypeDerived              -- Used for deriving instances of newtypes, where the
                                -- witness dictionary is identical to the argument 
                                -- dictionary.  Hence no bindings, no pragmas.
-       (Maybe [PredType])
-               -- Nothing      => The newtype-derived instance involves type variables,
-               --                 and the dfun has a type like df :: forall a. Eq a => Eq (T a)
-               -- Just (r:scs) => The newtype-defined instance has no type variables
-               --                 so the dfun is just a constant, df :: Eq T
-               --                 In this case we need to know waht the rep dict, r, and the 
-               --                 superclasses, scs, are.  (In the Nothing case these are in the
-               --                 dict fun's type.)
-               --                 Invariant: these PredTypes have no free variables
-               -- NB: In both cases, the representation dict is the *first* dict.
 
 pprInstInfo info = vcat [ptext SLIT("InstInfo:") <+> ppr (idType (iDFunId info))]
 
 pprInstInfoDetails info = pprInstInfo info $$ nest 2 (details (iBinds info))
   where
-    details (VanillaInst b _)  = pprLHsBinds b
-    details (NewTypeDerived _) = text "Derived from the representation type"
+    details (VanillaInst b _) = pprLHsBinds b
+    details NewTypeDerived    = text "Derived from the representation type"
 
 simpleInstInfoClsTy :: InstInfo -> (Class, Type)
 simpleInstInfoClsTy info = case instanceHead (iSpec info) of