Lets suppose I have a file that has encoded things of different
types as integers, and now I would like to convert them back
into specific instances of a data type.  For example, I have a
file that contains 1,1,2,3 and I would like the output to be
[Red, Red, Green, Blue]. I also would like to do this
generically, so that if I wanted to convert the same list of
integers into say Sizes, I would get [Small, Small, Medium,
Large]  Now please have a look at the following code:

{-# LANGUAGE DeriveDataTypeable #-}
import Data.Generics
data Color = Red | Green | Blue deriving (Eq,Ord,Read,Show,Typeable,Data)
data Size  = Small | Mediaum | Large deriving (Eq,Ord,Read,Show,Typeable,Data)