Improve handling of newtypes (fixes Trac 1495)

[ghc-hetmet.git] / compiler / types / TyCon.lhs

diff --git a/compiler/types/TyCon.lhs b/compiler/types/TyCon.lhs
index e1cf3f2..c495ebb 100644 (file)
--- a/compiler/types/TyCon.lhs
+++ b/compiler/types/TyCon.lhs
@@ -6,11 +6,11 @@
 The @TyCon@ datatype
 
 \begin{code}
-{-# OPTIONS_GHC -w #-}
+{-# 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/WorkingConventions#Warnings
+--     http://hackage.haskell.org/trac/ghc/wiki/Commentary/CodingStyle#Warnings
 -- for details
 
 module TyCon(
@@ -33,7 +33,7 @@ module TyCon(
        isEnumerationTyCon, isGadtSyntaxTyCon, isOpenTyCon,
        assocTyConArgPoss_maybe, isTyConAssoc, setTyConArgPoss,
        isTupleTyCon, isUnboxedTupleTyCon, isBoxedTupleTyCon, tupleTyConBoxity,
-       isRecursiveTyCon, newTyConRep, newTyConRhs, newTyConCo_maybe,
+       isRecursiveTyCon, newTyConRhs, newTyConEtadRhs, newTyConCo_maybe,
        isHiBootTyCon, isSuperKindTyCon,
         isCoercionTyCon_maybe, isCoercionTyCon,
         isImplicitTyCon,
@@ -250,31 +250,20 @@ data AlgTyConRhs
                                --  = the representation type of the tycon
                                -- The free tyvars of this type are the tyConTyVars
       
-        nt_co :: Maybe TyCon,   -- The coercion used to create the newtype
+        nt_co :: Maybe TyCon,   -- A CoercionTyCon used to create the newtype
                                 -- from the representation
-                                -- optional for non-recursive newtypes
+                                -- Optional for non-recursive newtypes
                                -- See Note [Newtype coercions]
+                               -- Invariant: arity = #tvs in nt_etad_rhs;
+                               --      See Note [Newtype eta]
+                               -- Watch out!  If any newtypes become transparent
+                               -- again check Trac #1072.
 
-       nt_etad_rhs :: ([TyVar], Type) ,
+       nt_etad_rhs :: ([TyVar], Type)
                        -- The same again, but this time eta-reduced
                        -- hence the [TyVar] which may be shorter than the declared 
                        -- arity of the TyCon.  See Note [Newtype eta]
-
-       nt_rep :: Type  -- Cached: the *ultimate* representation type
-                       -- By 'ultimate' I mean that the top-level constructor
-                       -- of the rep type is not itself a newtype or type synonym.
-                       -- The rep type isn't entirely simple:
-                       --  for a recursive newtype we pick () as the rep type
-                       --      newtype T = MkT T
-                       -- 
-                       -- This one does not need to be eta reduced; hence its
-                       -- free type variables are conveniently tyConTyVars
-                       -- Thus:
-                       --      newtype T a = MkT [(a,Int)]
-                       -- The rep type is [(a,Int)]
-                       -- NB: the rep type isn't necessarily the original RHS of the
-                       --     newtype decl, because the rep type looks through other
-    }                  --     newtypes.
+    }
 
 visibleDataCons :: AlgTyConRhs -> [DataCon]
 visibleDataCons AbstractTyCon                = []
@@ -333,7 +322,6 @@ data SynTyConRhs
 
 Note [Newtype coercions]
 ~~~~~~~~~~~~~~~~~~~~~~~~
-
 The NewTyCon field nt_co is a a TyCon (a coercion constructor in fact)
 which is used for coercing from the representation type of the
 newtype, to the newtype itself. For example,
@@ -397,6 +385,14 @@ we get:
        w2 = w1
 And now Lint complains unless Foo T == Foo [], and that requires T==[]
 
+This point carries over to the newtype coercion, because we need to
+say 
+       w2 = w1 `cast` Foo CoT
+
+so the coercion tycon CoT must have 
+       kind:    T ~ []
+ and   arity:   0
+
 
 Note [Indexed data types] (aka data type families)
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
@@ -466,6 +462,7 @@ data PrimRep
   | AddrRep            -- a pointer, but not to a Haskell value
   | FloatRep
   | DoubleRep
+  deriving( Eq )
 
 -- Size of a PrimRep, in bytes
 sizeofPrimRep :: PrimRep -> Int
@@ -805,7 +802,7 @@ tcExpandTyCon_maybe, coreExpandTyCon_maybe
                  Type,                 -- Body type (not yet substituted)
                  [Type])               -- Leftover args
 
--- For the *typechecker* view, we expand synonyms only
+-- For the *typechecker* view, we expand (closed) synonyms only
 tcExpandTyCon_maybe (SynTyCon {tyConTyVars = tvs, 
                               synTcRhs = SynonymTyCon rhs }) tys
    = expand tvs rhs tys
@@ -859,9 +856,7 @@ tyConFamilySize (AlgTyCon   {algTcRhs = DataTyCon {data_cons = cons}}) =
 tyConFamilySize (AlgTyCon   {algTcRhs = NewTyCon {}})                  = 1
 tyConFamilySize (AlgTyCon   {algTcRhs = OpenTyCon {}})                 = 0
 tyConFamilySize (TupleTyCon {})                                               = 1
-#ifdef DEBUG
 tyConFamilySize other = pprPanic "tyConFamilySize:" (ppr other)
-#endif
 
 tyConSelIds :: TyCon -> [Id]
 tyConSelIds (AlgTyCon {algTcSelIds = fs}) = fs
@@ -878,9 +873,9 @@ newTyConRhs :: TyCon -> ([TyVar], Type)
 newTyConRhs (AlgTyCon {tyConTyVars = tvs, algTcRhs = NewTyCon { nt_rhs = rhs }}) = (tvs, rhs)
 newTyConRhs tycon = pprPanic "newTyConRhs" (ppr tycon)
 
-newTyConRep :: TyCon -> ([TyVar], Type)
-newTyConRep (AlgTyCon {tyConTyVars = tvs, algTcRhs = NewTyCon { nt_rep = rep }}) = (tvs, rep)
-newTyConRep tycon = pprPanic "newTyConRep" (ppr tycon)
+newTyConEtadRhs :: TyCon -> ([TyVar], Type)
+newTyConEtadRhs (AlgTyCon {algTcRhs = NewTyCon { nt_etad_rhs = tvs_rhs }}) = tvs_rhs
+newTyConEtadRhs tycon = pprPanic "newTyConEtadRhs" (ppr tycon)
 
 newTyConCo_maybe :: TyCon -> Maybe TyCon
 newTyConCo_maybe (AlgTyCon {algTcRhs = NewTyCon { nt_co = co }}) = co