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) g = Green c = undefined :: Color s = undefined :: Size t = do print $ toConstr g -- Green print $ dataTypeOf c -- DataType {tycon = "Main.Color", datarep = AlgRep [Red,Green,Blue]} convert :: (Data a, Data b) =Int -a -b convert i x = let c = dataTypeConstrs (dataTypeOf x) !! (i-1) in fromConstr c I would like to be able to say: x = convert 1 c and have it assign Red to x then I would like to say: y = convert 1 s and have it assign Small to y, however, when I try that I get: Ambiguous type variable `b' in the constraint: `Data b' arising from a use of `convert' at <interactive>:1:8-18 Probable fix: add a type signature that fixes these type variable(s) Of course if I say x :: Color = convert 1 c, it works, but I would like to avoid that if possible, as all of the information is already contained in the parameter c. Is there any way to do this? Thanks in advance for your wise counsel. Best wishes, Henry Laxen