On 10/8/07, bjornie <bjorn.wikstrom@gmail.com> wrote:

Hi! I'm kind of new to functional programming with Haskell, I'm reading a university course you see. Now I'm having some problems with one of our lab assignments, we're supposed to write a program handling numeric operations (addition, multiplication, and some functionality like sin and cos).

I've made a data type; data Expr = Num Double | Add Expr Expr | Mul Expr Expr...

I.e for representing the expression: 2.4 * (4.0 + 1.5) The Haskell representation is: Mul (Num 2.4) (Add (Num 4.0) (Num 1.5))

My problem is to convert a String to an expression like the one above. I.e the String: "2.4*(4.0+1.5)"

If we would use plain Integers there would not be a problem, since the Prelude function 'digitToInt' :: Char -> Int does nearly what I would like to do. But I'm stuck on the Doubles! How can I solve this in a smooth way?

You can use read. Here's a session from GHC's interpreter: $ ghci Prelude> read "1.0" :: Double 1.0 Since read is polymorphic (i.e. works for different types) you need to specify the type of the result you want using ":: Double".