Hi Max, I've come into trouble defining function `gmap` which will work on these
data types:
data Row = Row (E Name) (E Salary) (E Department)
type E a = Either (Maybe RowIndex) (Maybe a)
type RowIndex = Int
`RowIndex`, `Name`, `Salary`, `Department` have kind *
pseudocode:
gmap :: (E a -> E a) -> Row -> Row
[...]
So, generic programming folks, is it even possible to define such function? I don't really care about using GHC extensions, I don't care about code being portable, I just want to remove boilerplate and prevent introducing bugs.
I'm not sure the problem you're running into is strictly a generic programming (GP) one. Typically, GP takes code that is often written and generalizes it, so that it doesn't have to be written for multiple datatypes. For your problem, I think the first issue is figuring out how to write the non-generic function. I don't know if this is exactly what you want, but you can write a version of gmap using GADTs and rank-2 types. I've simplified some types, but it should be easily transferable to your code. For example, change the String, Float, etc. to your Salary, Department, whatever. --- {-# LANGUAGE GADTs #-} {-# LANGUAGE Rank2Types #-} module Main where data T a where String :: T String Float :: T Float Integer :: T Integer data Row = Row (Maybe String) (Maybe Float) (Maybe Integer) deriving Show f :: T a -> Maybe a -> Maybe a f String (Just "a") = Just "z" f _ x = x gmap :: (forall a . T a -> Maybe a -> Maybe a) -> Row -> Row gmap f (Row x y z) = Row (f String x) (f Float y) (f Integer z) main = do print $ gmap f $ Row Nothing (Just 5.4) (Just 3) -- ==> Row Nothing (Just 5.4) (Just 3) print $ gmap f $ Row (Just "a") Nothing Nothing -- ==> Row (Just "z") Nothing Nothing --- If this is what you're looking for, then I think it might be possible to do this more generally, though I haven't looked into it. Regards, Sean