[Haskell-cafe] Polymorphic algebraic type constructors

[Duncan Coutts]

On Tue, 2004-06-22 at 20:52, Adrian Hey wrote:
> On Tuesday 22 Jun 2004 6:20 pm, MR K P SCHUPKE wrote:
> > ahh but in this example:
> >
> >  f :: [Int] -> [Bool]
> >  f (i:is) = even i : f is
> >  f e@[]   = e
> >
> > e is an empty list of Ints not an empty list of Bools!
> 
> If the difference is significant (I don't believe it is)
> then consistency demands that this expression should
> give a type error..
> 
>  let e=[] in (length e : e, null e : e)
> 
> It doesn't, so clearly e can be both an empty list of
> Ints and an empty list of Bools :-)

I think the point is that [] (or e in your example) has type
forall a.[a]
where as in the original example e was bound to an argument with the
type [Int], so e could not be used where something of type [Bool] was
required. On the other hand
[] :: forall a.[a]
could be used in either context.

Duncan