[Haskell-cafe] Applying different functions to different types for lists

[Szekeres István]

Hi,

I want to build a framework to apply different functions to different types
of lists and I want to generalize the solution as much as possible but my
Haskell knowledge is not really enough to do this (I'm a haskell novice).

The problem has two dimensions:

1. Lets say I have two functions:

r13, ul : Char -> Char

where r13 returns the rot13ed character, and ul converts upper case chars to
lower case and vice versa. Later I want a rot13 function that can operate on
different types of lists and an uxl function that can also operate on
different types of lists.

2. Lets say I have two kinds of lists to handle: String and
Data.ByteString.Lazy as L.
For Strings I want the updater function to be used for each character and
for L I want all items to be converted to Char, the updater function to be
updated and the result to be converted back to the original item type.

Therefore I define a common class for handling these lists:

class ListUpdater a where
    updateFn :: Char -> Char
    update :: a -> a

so I can define the update function for the different types of lists:

instance ListUpdater String where
    update = map updateFn

instance ListUpdater L.ByteString where
    update = L.pack . (map updateFn) . L.unpack


But now how should I glue these 2 dimensions together? How to say that I
want a rot13 function that applies to any data type instantiated from
ListUpdater and that is using the r13 function to do the update? And also
that I want an uxl function that also applies to any data type instantiated
from ListUpdater and that is using the ul function to do the update?

Thanks,
Istvan
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