[Haskell-cafe] Type Constraints on Data Constructors

[Guy]

Can this be extended to records, without redundant repetition?

data Baz f a = Baz {baz :: Foo f => f a, baz2 :: Foo f => f a}

The type constraint for baz2 adds no information, as it's the same f as baz, but I can't leave it out.



----- Original Message -----
> From: Daniel Schüssler <danlex at gmx.de>
> To: haskell-cafe at haskell.org
> Cc: Guy <guytsalmaves-h at yahoo.com>
> Sent: Thursday, 9 June 2011, 2:06
> Subject: Re: [Haskell-cafe] Type Constraints on Data Constructors
> 
> Hello,
> 
> you might be thinking of this type?
> 
> {-# LANGUAGE Rank2Types #-}
> 
> class Foo f where
>     foo :: a -> f a
> 
> data Baz f a = Baz (forall f. Foo f => f a) 
> 
> instance Foo (Baz f) where
>      foo a = Baz (foo a)
> 
> Maybe the difference between Bar and Baz ist best explained by writing it with 
> an explicit class dictionary for Foo:
> 
> {-# LANGUAGE Rank2Types #-} 
> 
> data FooDict f = FooDict { 
>         foo :: forall a. a -> f a 
>     }
> 
> data Bar f a = Bar (FooDict f) (f a) 
> 
> data Baz f a = Baz (FooDict f -> f a) 
> 
> fooDict_Baz :: FooDict (Baz f)
> fooDict_Baz = FooDict (\a -> Baz (\d -> foo d a)) 
> 
> -- fooDict_Bar :: FooDict (Bar f)
> -- fooDict_Bar = FooDict (\a -> Bar ? ?) 
> -- Doesn't work - you'd have to create a 'FooDict f' and a 
> 'f a' out of just 
> an 'a'
> 
> 
> 
> Cheers,
> Daniel