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

17 Aug 2011

      On Tue, Aug 16, 2011 at 04:44:15PM +0200, Ertugrul Soeylemez wrote:
...
Brent Yorgey  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