Editing List instance (section)

== Answer ==

First you have to find out, what you really want to do.

=== Define a custom type ===

If you do not need list functions, but e.g. just want to use <hask>String</hask> values as identifiers,
then I advise to just define a custom type:
<haskell>
    newtype Identifier = Identifier String
</haskell>

and write the instances you need for that type.

=== Keep working with the list (or container) type ===

If you need list functions,
then the wrapper approach means that you have to litter your code with calls to the wrapper constructor.
In this case you should stick to the original composed type.
The trick in the Prelude for the <hask>Show</hask> class is,
that <hask>listToX</hask> has a default implementation.

If this is not possible in your application then introduce a new class like
<haskell>
    class Element a where
        listToX :: [a] -> X
</haskell>
and define instances like
<haskell>
    instance Element Char where
        listToX = ...

    instance Element a => C [a] where
        toX = listToX
</haskell>
.

More generally, you can introduce a new class like
<haskell>
    class IsChar a where
        fromChar :: Char -> a
        toChar :: a -> Char
    instance IsChar Char where
        fromChar = id
        toChar = id
</haskell>
and then whenever you want to use the "<hask>a</hask>" type as a <hask>Char</hask>,
you just convert it from or to <hask>Char</hask> with <hask>fromChar</hask> or <hask>toChar</hask>, respectively.

=== FlexibleInstances ===

If none of the above solution is appropriate for you,
then you may choose the <code>FlexibleInstances</code> language extension
that is available in GHC and Hugs.
See [https://downloads.haskell.org/~ghc/8.0.2/docs/html/users_guide/glasgow_exts.html#instance-termination-rules instance rules].

<haskell>
    {-# LANGUAGE FlexibleInstances #-}
    instance C [Char] where toX  = ...
</haskell>