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

[Ertugrul Soeylemez]

Brandon Allbery <allbery.b at gmail.com> wrote:

> I think what you're reaching for here is a GADT:
>
> > data BasicSelect a where
> >   SelectionNum :: Num a => a -> BasicSelect a
> >   SelectionStr :: Show a => a -> BasicSelect a
>
> I think you otherwise end up with a typeclass or with existentials;
> both introduce complexity, and I think the existentials solution still
> allows anything to be stuffed into it, whereas you want to limit it to
> SelectionNum or SelectionStr.  (Typeclasses at least require an
> instance to be defined first; but they don't prohibit people from
> doing so, if that's what you're after.)

That's a bit of a contradiction, because you are using existentials
yourself in your GADT.


Greets,
Ertugrul


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