[Haskell-cafe] Figuring out if an algebraic type is enumerated through Data.Generics?

Edward Kmett ekmett at gmail.com
Tue May 6 13:42:33 EDT 2008

On Tue, May 6, 2008 at 12:34 PM, Alfonso Acosta
<alfonso.acosta at gmail.com> wrote:

| So, the question is. Is there a way to figure out the arity of data
| constructors using Data.Generics ?

| I'm totally new to generics, but (tell me if I'm wrong) it seems that
| Constr doesn't hold any information about the data-constructor
| arguments. Why is it so?


Playing around with it, I was able to abuse gunfold and the reader
comonad to answer the problem :

fst $ (gunfold (\(i,_) -> (i+1,undefined)) (\r -> (0,r)) (toConstr
"Hello") :: (Int,String))

returns 2, the arity of (:), the outermost constructor in "Hello"

A longer version which does not depend on undefined would be to take
and define a functor that discarded its contents like:

> module Args where

> import Data.Generics

> newtype Args a = Args { runArgs :: Int } deriving (Read,Show)

> tick :: Args (b -> r) -> Args r
> tick (Args i) = Args (i + 1)

> tock = const (Args 0)

> argsInCons = runArgs $ (gunfold tick tock (toConstr "Hello") :: (Args String)

Basically all I do is rely on the fact that gunfold takes the 'tick'
argument and calls it repeatedly for each argument after a 'tock' base

The use of the reader comonad or functor is to give gunfold a
'functor-like' argument to meet its type signature.

-Edward Kmett

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