newtype and existentials

[Simon Peyton-Jones]

yes.  a newtype declares a new type isomorphic to an existing type.  
	newtype T = MkT S
declares T to be isomorphic to S.

There is no existing Haskell type isomorphic to your Dynamic.

In concrete terms, the newtype constructor is discarded before we get to
System F; but we can't do that with an existential.

S

| -----Original Message-----
| From: glasgow-haskell-bugs-bounces at haskell.org
[mailto:glasgow-haskell-bugs-
| bounces at haskell.org] On Behalf Of Ralf Hinze
| Sent: 18 November 2005 13:28
| To: glasgow-haskell-bugs at haskell.org
| Subject: newtype and existentials
| 
| Is there a strong reason why
| 
| > {-# OPTIONS -fglasgow-exts #-}
| > data Type :: (* -> *) where
| >   Int   ::  Type Int
| > data Dynamic  =  forall a . Dyn (a, Type a)
| 
| works, but not
| 
| > newtype Dynamic  =  forall a . Dyn (a, Type a)
| 
| Cheers, Ralf
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