Unlifted data types

[Eric Seidel]

Another good example would be

  foo :: ![Int] -> ![Int]

Does this force just the first constructor or the whole spine? My guess
would be the latter.

On Fri, Sep 4, 2015, at 08:43, Edward Z. Yang wrote:
> 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