how frowned-upon is "recursive coreView"?

[Simon Peyton-Jones]

The only reason this is bad is that in GHC a function only has one type.  So suppose 
	f :: forall a. Eq a => a -> a
(which is just surface syntax for
	ForAllTY a (FunTy (PredTy (ClassP Eq [a])) ...)
)

If you change that to
	f :: forall a. DEq a -> a -> a
where DEq is the data type of the dictionary rep for (Eq a) predicate,
then everything will be fine except that module that *import* this module won't realise that the DEq parameter is a type-class parameter.  The PredTy stuff serves to tell the type checker which parameters are implict to be elaborated by type inference

S

| -----Original Message-----
| From: cvs-ghc-bounces at haskell.org [mailto:cvs-ghc-bounces at haskell.org] On
| Behalf Of Adam Megacz
| Sent: 18 February 2011 22:48
| To: cvs-ghc at haskell.org
| Subject: how frowned-upon is "recursive coreView"?
| 
| 
| The function at the end of this email walks over a Type, basically
| (recursivey) replacing it with its coreView.  The resulting tree has no
| PredTy's except for (PredTy (EqPred _ _)).
| 
| How problematic is it for a core-to-core translation to replace all the
| Type's in an Expr with their coreView-ification this way?  Is it likely
| to cause compilation/core-lint failures, or is it just ugly?
| 
| I suppose that at the very least it makes it harder to debug passes
| which happen after the coreView-ifying pass.  And I realize that
| actually replacing a type with its coreView (rather than calling
| coreView "on demand") is not the preferred way of doing things.
| 
| Thanks,
| 
|   - a
| 
| coreViewDeep :: Type -> Type
| coreViewDeep t =
|     case t of
|       TyVarTy tv       -> TyVarTy tv
|       FunTy arg res    -> FunTy (coreViewDeep arg) (coreViewDeep res)
|       AppTy fun arg    -> AppTy (coreViewDeep fun) (coreViewDeep arg)
|       ForAllTy fun arg -> ForAllTy fun (coreViewDeep arg)
|       TyConApp tc tys  -> let t' = TyConApp tc (map coreViewDeep tys)
|                         in case coreView t' of
|                                Nothing     -> t'
|                                Just    t'' -> t''
|       PredTy p         -> case coreView t of
|                           Nothing     -> PredTy p
|                           Just    t'  -> t'
| 
| 
| 
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