[Haskell] Litle problem with a haskell Function...

Hello, I' a Haskell Newbie and i need some help about a function.. My problem is to update a list of values in Runtime. For example i have a initial list: list1=3D"a"=20 and a function: add::add::String->[String] add x =3D [list1] ++ [x]=20 I need to do=20 add "c"=20 and the "c" most be append to list1 to make list1=3D["a","c"] Then if i input list1 the output must be ["a","c"] But i can't do it... Could you help me ?!