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

[Markus Läll]

Hi and thank you for the answers -- I'll use Data for now since generics
seem scary at this point.

Another question :): is there a better way to get the constructor of some
partially applied data constructor. My current solution follows (and it
works), but I'm wondering if there is a better way? The issue is that I'm
feeding `undefined`s until the constructor is saturated and then get the
Constr, but it feels like using a loophole (the existence of `undefined`)
to achieve the result.

class GetConstr a where
  getConstr :: a -> Constr
instance {-# OVERLAPPING #-} (GetConstr b) => GetConstr (a -> b) where
  getConstr f = getConstr (f undefined)
instance {-# OVERLAPPABLE #-} (Data a) => GetConstr a where
  getConstr a = toConstr a


On Mon, Oct 29, 2018 at 11:07 PM Artem Pelenitsyn <a.pelenitsyn at gmail.com>
wrote:

> 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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-- 
Markus Läll
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