[Haskell-beginners] Monad question

[mike h]

Hi,
This isn’t homework! I’ve been staring at this for several hours - and that usually works.
I also tried typed holes to no avail.  I thinks it is  quite simple really but I’ve gone past seeing it!

I have 
data Expr a = Var a | Add (Expr a) (Expr a) 

and would like to write

convert :: Expr (Maybe a) -> Maybe (Expr a)

which returns Nothing if there is an occurrence of Nothing inside the
input expression e, otherwise it returns Just e', where e'
is a new expression where the internal values of type a are not wrapped in Just.
You should use the functionality of the Maybe monad to implement
the convert function. 


Thanks 
Mike