type OtherType = { x :: Int, y :: Int, z :: String } deriving (Show)

data SelectionType = A | B

myMapper = map foo someMyTypes

foo :: SelectionType -> MyType -> OtherType
foo sel t = [ x t, y t, z t <> something ]
  where
    case SelectionType of
      A -> something = "a selected"
      B -> something = "b selected"

foo sel t = [ x t, y t, z t <> something ]
  where
    something = case sel of
      A -> "a selected"
      B -> "b selected"