RE: Core, shadowing type variable names, possible fix

Right; I've committed this change.

SImon

| -----Original Message-----
| From: Tobias Gedell [mailto:d99getob@xxxxxxxxxxxxxxxx]
| Sent: 29 January 2003 18:08
| To: glasgow-haskell-bugs
| Subject: Core, shadowing type variable names, possible fix
| 
| I have found what causes the bug and implemented a possibly fix for
it.
| The problem has to do with source types that aren't expanded before
| tidying them.
| 
| Here is a small example where the error occurs:
| 
| -----
| module Test where
| 
| newtype A a = A (forall b. b -> a)
| 
| test :: forall q b. q -> A b
| test _ = undefined
| -----
| 
| 
| When generating Core for this program we get:
| 
| -----
| %module Test
|    %newtype Test.A a = %forall b . b -> a;
|    Test.A :: %forall a . (%forall b . b -> a) -> %forall b . b -> a =
|      %note "InlineMe"
|      \ @ a (tpl::%forall b . b -> a) -> tpl;
|    Test.zdgfromA :: %forall a . (%forall b . b -> a) ->
|                              %forall b . b -> a =
|      \ @ a (g::%forall b . b -> a) -> g;
|    Test.zdgtoA :: %forall a . (%forall b . b -> a) ->
|                            %forall b . b -> a =
|      Test.A;
|    Test.test :: %forall q b . q -> %forall b . b -> b =
|      \ @ q @ b (ds::q) -> GHCziErr.undefined @ (%forall b . b -> b);
| -----
| 
| Here the type for Test.test is wrong since the second binding of b
| shadows the first binding of b. We do also pass the wrong type to
| GHCziErr.undefined.
| 
| The tidyXXX functions should take care of this and rename the second b
| to b1. After diving into GHC I found that the problem is that the
types
| and expressions are tidied before the source types are expanded. When
| generating external core the source types are expanded first in
make_ty
| in MkExternalCore.lhs.
| 
| 
| I believe that there are three possible fixes:
| 
|   1. Make sure that all type variables that are bound in non recursive
| newtypes are made unique
|   2. Expand all non recursive newtypes before using tidyXXX
|   3. Not expand the non recursive newtypes when generating external
core
| 
| 
| I have implemented the third alternative. I changed the function
make_ty
| in MkExternalCore.lhs to:
| 
| -----
| make_ty :: Type -> C.Ty
| make_ty (TyVarTy tv) = C.Tvar (make_var_id (tyVarName tv))
| make_ty (AppTy t1 t2) = C.Tapp (make_ty t1) (make_ty t2)
| make_ty (TyConApp tc ts) = foldl C.Tapp (C.Tcon (make_con_qid
(tyConName
| tc))) (map make_ty ts)
| make_ty (FunTy t1 t2) = make_ty (TyConApp funTyCon [t1,t2])
| make_ty (ForAllTy tv t) = C.Tforall (make_tbind tv) (make_ty t)
| make_ty (SourceTy (NType tc ts)) = foldl C.Tapp (C.Tcon (make_con_qid
| (tyConName tc))) (map make_ty ts)
| make_ty (SourceTy p) = make_ty (sourceTypeRep p)
| make_ty (NoteTy _ t) = make_ty t
| -----
| 
| 
| Now the generated Core for the program listed above becomes:
| 
| -----
| %module Test
|    %newtype Test.A a = %forall b . b -> a;
|    Test.A :: %forall a . (%forall b . b -> a) -> Test.A a =
|      %note "InlineMe"
|      \ @ a (tpl::%forall b . b -> a) -> tpl;
|    Test.zdgfromA :: %forall a . Test.A a -> %forall b . b -> a =
|      \ @ a (g::Test.A a) -> g;
|    Test.zdgtoA :: %forall a . (%forall b . b -> a) -> Test.A a =
|      Test.A;
|    Test.test :: %forall q b . q -> Test.A b =
|      \ @ q @ b (ds::q) -> GHCziErr.undefined @ (Test.A b);
| -----
| 
| 
| 
| 
| Sincerely,
|   Tobias
| 
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