タプルに対するgenericな加算（mkMとgmapMの使い方）

Generics.hs

module Main where
import Control.Monad.Writer (Writer, execWriter, tell)
import Data.Monoid (Sum(..))
import Data.Generics (Typeable, Data, gmapM, mkM)
import Data.Ratio (Ratio, (%))

main :: IO ()
main = do
  -- このタプルに対してgenericな加算を行う
  let tuple = ( 1 :: Int
              , 2 :: Integer
              , 3 :: Double
              , 4 :: Ratio Integer
              , 5 :: Int
              , 6 :: Integer
              , 7 :: Double
              )
      genericSum :: (Num b, Typeable b, Data a) => a -> Writer (Sum b) a
      genericSum = gmapM (mkM $ \x -> tell (Sum x) >> return x)

  print (getSum $ execWriter $ genericSum tuple :: Int)
  print (getSum $ execWriter $ genericSum tuple :: Integer)
  print (getSum $ execWriter $ genericSum tuple :: Double)
  print (getSum $ execWriter $ genericSum tuple :: Ratio Integer)

gistfile1.txt

Prelude> :main
6
8
10.0
4 % 1