[Haskell-cafe] Avoiding parametric function binding

[Kevin Quick]

On Sat, 31 Dec 2011 08:50:05 -0700, Stephen Tetley  
<stephen.tetley at gmail.com> wrote:

> Maybe you want a deconstructor (sometime called an eliminator)?
>
> deconsVar :: (Maybe Int -> a) -> (Maybe String -> a) -> Var -> a
> deconsVar f g (V1 a) = f a
> deconsVar f g (V2 b) = g b

That works and has the advantage of allowing a single deconstructor  
definition that can be reused in multiple places, but it requires me to  
add an argument for each wrapped type, which causes some ripple effect if  
the type changes.

Also, if that argument is parametric over the possible inputs (as asserted  
by a class restriction) then it starts to get a bit tedious:

main = putStrLn . deconsVar elemStr elemStr $ test

If I find myself adding a V3 and a V4 to my datatype in the future then  
that would change to:

main = putStrLn . deconsVar elemStr elemStr elemStr elemStr $ test

I was hoping to find a way that would let the functor argument retain its  
parametricity and therefore not need such repetition.

-Kevin

-- 
-KQ