Editing List instance (section)

== Question ==

How to make a list type an instance of some type class in Haskell 2010?
Haskell 2010 does not support instances on particular composed types like <hask>String</hask>.


If I have a type class for conversion to a type <hask>X</hask>:

<haskell>
    class C a where
        toX   :: a -> X
</haskell>
 
I can define instances for 

<haskell>
    instance C Int where toX  = ...
    instance C Double where toX  = ...
    instance C Tuple where toX  = ...
</haskell>
 
but not for Strings, given that they are a synonym for <hask>[Char]</hask>. Hence:

<haskell>
    instance C String where toX  = ...
</haskell>
 
results in:

<code>
    Illegal instance declaration for `C String'
        (The instance type must be of form (T a b c)
         where T is not a synonym, and a,b,c are distinct type variables)
    In the instance declaration for `C String'
</code> 
 
Is there some type class cleverness that can make this work in Haskell 98?

I'm aware of the approach taken by class <hask>Show</hask> in the prelude,
which adds a extra method to the class:
<haskell>
    class C a where
        toX     :: a -> X
        listToX :: [a] -> X
</haskell>
 
but I believe this says that whenever we can convert a to an <hask>X</hask> we can also
convert <hask>[a]</hask> to an <hask>X</hask>, whereas I only want <hask>[Char]</hask> to be acceptable.