[clean-list] Parametrizing instance declarations

[David C. Norris]

How might one define the following three class instances without 
duplicating code?  My attempts to parametrize the declaration fail with 
errors such as

(1) type variable of type of lifted argument b appear in the specified type
(2) internal overloading of "_createArray" could not be solved.

----------------------------------------------

class copyElt e :: {#e} Int -> e

instance copyElt Char
where
    copyElt a i = c.[0]
    where
        c :: {#Char}
        c = {a.[i]}

instance copyElt Int
where
    copyElt a i = c.[0]
    where
        c :: {#Int}
        c = {a.[i]}

instance copyElt Real
where
    copyElt a i = c.[0]
    where
        c :: {#Real}
        c = {a.[i]}

----------------------------------------------

What I would hope to accomplish is something like:

instance copyElt e | e <- [Char, Int, Real] // bogus syntax
where
    copyElt a i = c.[0]
    where
        c :: {#e}
        c = {a.[i]}

Regards,
David