evFilter :: [ErrVal a] -> [a]
evFilter = foldr f []
  where
    f (EValue a) as = a:as
    f _ as= as
evFilter :: [ErrVal a] -> [a]
evFilter = foldr f []
  where
    f (EValue a) as = a:as
    f _ as= as
evFilter = foldr f []
  where
    f (EValue a) as = a:as
    f _ as= as