[Haskell-beginners] Question: Data Type for user selection

[Brent Yorgey]

On Tue, Aug 16, 2011 at 04:44:15PM +0200, Ertugrul Soeylemez wrote:
> Brent Yorgey <byorgey at seas.upenn.edu> wrote:
> 
> > > That's a bit of a contradiction, because you are using existentials
> > > yourself in your GADT.
> >
> > No, he isn't.
> >
> >    data BasicSelect a where
> >      SelectionNum :: Num a => a -> BasicSelect a
> >      SelectionStr :: Show a => a -> BasicSelect a
> >
> > 'a' shows up in the result type of both constructors, so there is no
> > existential quantification going on here.
> 
> Oh, right.  How would one express this as an ADT?  Seems impossible to
> me.

You cannot, with just Haskell 2010.  Strangely, if you try this:

  data BasicSelect a = Num a  => SelectionNum a 
                     | Show a => SelectionStr a

you get this error (ghc 7.0.3):

  Data constructor `SelectionNum' has existential type variables, or a
  context
    (Use -XExistentialQuantification or -XGADTs to allow this)
  In the definition of data constructor `SelectionNum'
  In the data type declaration for `BasicSelect'

And enabling ExistentialQuantification makes the error go away!  So
apparently the ExistentialQuantification flag also enables type class
constraints on data constructors, even when no existential
quantification is involved.  Odd.

-Brent