Unlifted data types

[Edward Z. Yang]

Hello Eric,

You can't tell; the head not withstanding, `[a]` is still a lazy list,
so you would need to look at the function body to see if any extra
forcing goes on. `Force` does not induce `seq`ing: it is an obligation
for the call-site.

(Added it to the FAQ).

Edward

Excerpts from Eric Seidel's message of 2015-09-04 09:06:15 -0700:
> 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