[Haskell] [Haskell-beginners] Restrict type in phantom data-type

[Baa]

David, hello!

1. Is it the same/different as:

  data family Day a
  data Sunny
  data Rainy
  data instance Day Sunny = SunnyDay deriving Show
  data instance Day Rainy = RainyDay deriving Show

  ..and here you can not create `Day Int` object because no `Day Int`
  constructor (but you can create such constructor)

? Or in case with type families there is possibility to extend it to
`Day Int` and in case with DayaKinds it's totally impossible?

2. I read somewhere (on forums) that restrictions on data types... I
don't remember exactly, but something like they are not real
restrictions or are related to old extension which is/will be
deprecated. I'm not sure. Also, I'm not sure is it - in your example -
restriction (constraint) or something else. Am I wrong?

> This is maybe edging toward haskell-cafe territory, but you can
> definitely do this in haskell.
> 
> {-# LANGUAGE DataKinds, KindSignatures #-}
> 
> data DayType = Sunny | Rainy
> 
> data Day (a :: DayType) = Day
> 
> 
> sunnyDay :: Day Sunny
> sunnyDay = Day
> 
> rainyDay :: Day Rainy
> rainyDay = Day
> 
> -- impossibleDay :: Day ()
> -- impossibleDay = Day
> 
> On Fri, Sep 1, 2017 at 10:18 AM, Baa <aquagnu at gmail.com> wrote:
> > Hello, List!
> >
> > For example, I have specialized (right nameis phantom?) type:
> >
> >   data Day a = Day { ... no `a` here }
> >   data Sunny
> >   data Rainy
> >
> >   joyToday :: Day Sunny -> IO ()
> >   joyToday day = ...
> >
> >   melancholyToday :: Day Rainy -> IO ()
> >   melancholyToday day = ...
> >
> > And I can create (in spite of that it's phantom) some day:
> >
> >   let day1 = Day {...} :: Day Sunny
> >   joyToday day1
> >
> > but no problem to create `Day Int`, `Day Char`, etc which is
> > pointless actually (sure "creator"-function can be exported from the
> > module only, but I'm talking about type-level solution).
> >
> > I know that constraints (`... =>`) on data types are
> > redundant/removed from the language. And I'm not sure how it's
> > possible to restrict that parameter `a` (I know that it's possible
> > to Java/C++/Perl6 (not sure), some other languages but how to add
> > such restriction in Haskell? IMHO type families can help but I'm
> > not sure how it will look (Sunny, Rainy are "nullary" type, so...).
> >
> > Is it possible for Haskell too?
> >
> > ===
> > Best regards, Paul
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