[Haskell-cafe] Type Constraints on Data Constructors

[Daniel Schüssler]

Correction: I meant

data Baz f a = Baz (Foo f => f a)

(Dropped the 'forall', which would make the inner 'f' have nothing to do with 
the type parameter 'f' of 'Baz')

On 2011-June-09 Thursday 01:07:13 Daniel Schüssler wrote:
> 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
> 
> On 2011-June-08 Wednesday 20:45:56 Guy wrote:
> > {- continuing discussion from beginners@ -}
> > 
> > I have code such as
> > 
> > class Foo f where
> > 
> >      foo :: a -> f a
> > 
> > data Bar f a = Foo f => Bar {bar :: f a}
> > 
> > instance Foo (Bar f) where
> > 
> >      foo a = Bar $ foo a
> > 
> > GHC insists that I put Foo f => on the instance declaration, even though
> > the constructor for Bar implies this.
> > 
> > Is there any reason why GHC cannot infer this constraint from the Bar
> > constructor? One issue raised in the beginners thread is that
> > undefined :: Bar f a
> > is not Foo f, but as undefined cannot be evaluated, this would not appear
> > to be a problem.
> > 
> > 
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> 
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