How to override a default value, via lenses, only if incoming value is not Nothing

Question

I'm basically trying to override a bunch of default values in a record only if the user-specific values are NOT Nothing. Is it possible to do it via lenses?

import qualified Data.Default as DD

instance DD.Def Nouns where
  def  = Nouns
    {
      -- default values for each field come here
    }

lookupHStore :: HStoreList -> Text -> Maybe Text

mkNounsFromHStoreList :: HStoreList -> Nouns
mkNounsFromHStoreList h = (DD.def Nouns)
  & depSingular .~ (lookupHStore h "dep_label_singular")
  -- ERROR: Won't compile because Text and (Maybe Text) don't match

Answer 1

You could make your own combinator:

(~?) :: ASetter' s a -> Maybe a -> s -> s
s ~? Just a  = s .~ a
s ~? Nothing = id

Which you can use just like .~:

mkNounsFromHStoreList :: HStoreList -> Nouns
mkNounsFromHStoreList h =
  DD.def
    & myNoun1 ~? lookupHStore h "potato"
    & myNoun2 ~? lookupHStore h "cheese"