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

[Hartmut]

All,
thank You All for your gentle help. Now I am a step further :-) But there
raises up the next question:
In the last line, I want the datatype MultiSelect being limited to a's which
are of type "ExtendedSelect x".
How can I add this contraint?
Hartmut

{-# LANGUAGE GADTs #-}

module SelectionCriterias2 where

data InclusiveOrExclusive = Inclusive | Exclusive

-- 1. BasicSelect ------------------------------------------
data BasicSelect a where
  NumSelect :: Num a => a->BasicSelect a
  ShowSelect :: Show a => a->BasicSelect a

-- examples:
x1 = NumSelect 10
x2 = ShowSelect "Hello"
x3 = NumSelect 120.1

-- 2. ExtendedSelect ---------------------------------------
data ExtendedSelect a = ExtendedSelect {
  basicSel :: BasicSelect a,
  inclOrExcl :: InclusiveOrExclusive
}

-- examples:
x1i :: ExtendedSelect Integer
x1i = ExtendedSelect { basicSel = x1, inclOrExcl = Inclusive }
x1e = ExtendedSelect { basicSel = x1, inclOrExcl = Exclusive }
x2i = ExtendedSelect { basicSel = x2, inclOrExcl = Inclusive }
x2e = ExtendedSelect { basicSel = x2, inclOrExcl = Exclusive }

-- Abbreviation/helper for the construction:
extsel :: BasicSelect a -> InclusiveOrExclusive -> ExtendedSelect a
extsel s ie = ExtendedSelect { basicSel = s, inclOrExcl = ie }

-- examples:
x3i = extsel x3 Inclusive
x3e = extsel x3 Exclusive

-- 3. MultiSelect -----------------------------------------
data MultiSelect a = EmptySel | SingleSel a | MultiSel [a]




On Wed, Aug 17, 2011 at 3:42 PM, Brent Yorgey <byorgey at seas.upenn.edu>wrote:

> 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
>
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