[Haskell-beginners] if statements

[John Moore]

Hi
   I'm trying to get a if statement to work but not sure if this is the
right approach, what I want to do in simple english is to evaluate  one
expression at a time for example. (3+2)+(4+5) I want it to return 5 + (4+5)
then 5+9.

data Expression = Val Integer
                | Add Expression Expression
                | Subtract Expression Expression
                | Multiply Expression Expression
                | Divide Expression Expression
         deriving Show
evalStep :: Expression ->  Expression
evalStep (Val x)=  (Val x)

evalStep (Add x y) = do

  if x = isDigit
           then if y isDigit
             else evalStep x,y

I thinking about using recursion down a tree left then right but that seems
very complicated.
Any thoughts welcome!
John
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