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

[Artem Pelenitsyn]

Oh, didn't know about `one-liner`. This looks v. nice. Thank you!

-- Artem

On Mon, 29 Oct 2018 at 17:52 Li-yao Xia <lysxia at gmail.com> wrote:

> On 10/29/18 4:42 PM, Artem Pelenitsyn wrote:
> > 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.
>
> Although GHC.Generics has its shortcomings (usage complexity and compile
> times), I still find it worthwhile to advocate as a statically-typed
> alternative to the dynamically-typed Data.Data for many problems,
> including this one.
>
> Using the one-liner library (which is built around GHC.Generics), the
> equivalent line of code is:
>
>      getSum . gfoldMap @AnyType (const 1) :: T -> Int
>
> Data.Data is more visible mainly because it comes with a lot of
> functionality baked into the standard library, whereas GHC.Generics
> provides only a minimal interface and we have to find everything else in
> separate packages. However, there is no fundamental reason why one is a
> better fit than the other for the task of counting constructor fields.
>
> Li-yao
>
> >
> > On Mon, 29 Oct 2018 at 16:35 Li-yao Xia <lysxia at gmail.com
> > <mailto: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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