[Haskell-cafe] Get number of fields for non-record ADTs using Data type class?

[Artem Pelenitsyn]

I don't think there is a point in looking for GHC.Generics-based solution,
as Data.Data is the exact match for this kind of problem.

-- 
Best, Artem

On Mon, 29 Oct 2018 at 16:35 Li-yao Xia <lysxia at gmail.com> wrote:

> This maps every field to 1, and folds them together using (+):
>
>      Data.Data.gmapQl (+) 0 (const 1) :: T -> Int
>
> (There has to be a similarly easy solution using GHC.Generics instead
> but I can't think of one...)
>
> Li-yao
>
> On 10/29/18 2:56 PM, Markus Läll wrote:
> > Dear list,
> >
> > Is it possible te get the number of fields for data constructors for a
> > plain ADT, i.e something with no record fields? E.g for
> >
> > data T = A Int Double | B String (Maybe String)
> >
> > it would give 2 for both `A` and `B`.
> >
> > For a record it's possible using the `constrFields` function from
> Data.Data.
> >
> > I was trying to follow this tutorial by Christopher Done
> > https://chrisdone.com/posts/data-typeable, and I feel that it must be
> > possible somehow to get these numbers with the gmap*/gfold* functions,
> > but the use of them is over my head at the moment.
> >
> >
> > Best,
> >
> >
> >
> > --
> > Markus Läll
> >
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