Unlifted data types

[Edward Z. Yang]

Excerpts from Eric Seidel's message of 2015-09-04 08:29:59 -0700:
> You mention NFData in the motivation but then say that !Maybe !Int is
> not allowed. This leads me to wonder what the semantics of
> 
>   foo :: !Maybe Int -> !Maybe Int
>   foo x = x
> 
>   bar = foo (Just undefined)
> 
> are. Based on the FAQ it sounds like foo would *not* force the
> undefined, is that correct?

Yes. So maybe NFData is a *bad* example!

> Also, there's a clear connection between these UnliftedTypes and
> BangPatterns, but as I understand it the ! is essentially a new type
> constructor. So while
> 
>   foo1 :: !Int -> !Int
>   foo1 x = x
> 
> and
> 
>   foo2 :: Int -> Int
>   foo2 !x = x
> 
> have the same runtime behavior, they have different types, so you can't
> pass a regular Int to foo1. Is that desirable?

Yes.  Actually, you have a good point that we'd like to have functions
'force :: Int -> !Int' and 'suspend :: !Int -> Int'. Unfortunately, we
can't generate 'Coercible' instances for these types unless Coercible becomes
polykinded.  Perhaps we can make a new type class, or just magic
polymorphic functions.

Edward