RE: Using existential types

Thanks for the comments. I think your solution misses one point in my original example... I wrote:

data ColDesc rowv = forall a. ColDesc (rowv -> a) ([a] -> a) (a -> String)

calculate :: [rowv] -> ColDesc rowv -> ([String],String) calculate rs (ColDesc valf sumf fmtf) = let vals = map valf rs in (map fmtf vals, (fmtf.sumf) vals)

This code only does the (rowv -> a) evaluation once, whereas olegs version:

calculate :: [rowv] -> ColDesc rowv -> ([String],String) calculate rs (ColDesc fmtf sumf) = (map fmtf rs, sumf rs)

does the evaluation (rowv -> a) for each element twice, I think. In a real system, as opposed to my toy example, this computation is expensive, and I don't want to do it more that necessary. Tim