[Haskell-beginners] if statements

[Daniel Fischer]

Am Samstag 23 Januar 2010 18:40:10 schrieb John Moore:
> Hi
>    I'm trying to get a if statement to work but not sure if this is the
> right approach,

I think pattern matching and case .. of would be better suited.

> 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
>

-- evaluate one step only
evalStep (Add x y)
    = case x of
        (Val v) -> case y of
                     (Val w) -> Val (v+w)
                     other -> Add x (evalStep y)
        other -> Add (evalStep x) y
evalStep (Subtract x y) ... -- analogously, also Multiply and Divide

evalStep e = e -- only Val left.

Voilà, the leftmost innermost compound expression is reduced if the 
expression isn't fully evaluated. To completely evaluate:

eval :: Expression -> Expression
eval e@(Val _) = e
eval e = eval (evalStep e)

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