[Haskell-beginners] if statements

[Roel van Dijk]

On Sat, Jan 23, 2010 at 6:40 PM, John Moore <john.moore54 at gmail.com> wrote:
> 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


I renamed your Expression data type to simply E. Saves some typing :-)

data E = Val Integer
       | Add E E
       | Sub E E
       | Mul E E
       | Div E E
         deriving Show

evalStep :: E ->  E
evalStep (Val x) = (Val x)
-- Note that with a pattern match you can look deeply into a structure.
-- in this case we match on the addition of two values.
evalStep (Add (Val a) (Val b)) = Val (a + b)
evalStep (Add x@(Val _) y) = Add x (evalStep y)
evalStep (Add x y@(Val _)) = Add (evalStep x) y

You can copy the equations with "Add" and substitute with Sub, Mul and
Div and their related functions (-), (*) and div to get the complete
evalStep function.


Regards,
Roel