Hi. I have the following data type
import Data.Monoid import Control.Monad.Writer
data Graph = Graph {graphTitle :: String, graphPoints :: [Int]} deriving (Show) emptyGraph :: Graph emptyGraph = Graph {graphTitle = "", graphPoints = []}
Then i want to initialize it in Writer monad using (Dual (Endo Graph)) as monoid:
type GraphM t = WriterT (Dual (Endo Graph)) t
myGraph :: Monad t => GraphM t [Int] myGraph = do tell (setTitle "ab") tell (setTitle "ABCD") tell (modifyPoints (1 :)) tell (modifyPoints (2 :)) return [1, 2]
and then i can use it e.g. like
getGraph :: Monad t => (GraphM t a) -> t Graph getGraph m = do g <- execWriterT m return (appEndo (getDual g) emptyGraph)
printGraph :: IO () printGraph = getGraph myGraph >>= print
And to make this work i need two functions implemented for each record of Graph: get :: Graph -> a set :: a -> Graph -> Graph I can define per-record instances of them, like
setTitle :: String -> Dual (Endo Graph) setTitle x = Dual . Endo $ \g -> g{graphTitle = x}
modifyPoints :: ([Int] -> [Int]) -> Dual (Endo Graph) modifyPoints f = Dual . Endo $ \g -> let xs = graphPoints g in g{graphPoints = f xs}
but if Graph has many records, for most of them these 'set' functions ('get' functions i'll have from records) will look very similar. Is there a way how i can define all of them "in one row", i.e. using some generic 'set' implementation to which i should only pass e.g. record name or smth else? I.e. so, that above myGraph will look like .. tell(set r x) .. where r is record name (or smth else referencing particulat record) and x is value.