lends stuff

maybefold.hs

import Control.Lens
import Control.Monad.State
import Data.Foldable (mapM_)

data Rec =
    Rec { _recA :: Int, _recB :: String, _recC :: Double }
    deriving (Eq, Ord, Show)
makeLenses Rec

a = Just 1
b = Nothing
c = Just 3.14
x = Rec 0 "" 0.0

-- I want to update x with a, b, c  and skip Nothing.

main = do
  let x' = flip execState x $ do
    mapM_ (assign recA) a
    mapM_ (assign recB) b
    mapM_ (assign recC) c
  print x'


-- http://hpaste.org/80514#a80515