[Haskell-cafe] Re: [Haskell] Converting Double from String

(moving to haskell-cafe...) On 10/8/07, Johan Tibell wrote:

On 10/8/07, bjornie wrote:

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Hi! I'm kind of new to functional programming with Haskell, I'm reading

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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

a like

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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".

...although if you have something like Num (read x), you don't need the :: Double since the typechecker can figure it out (the Num constructor has to take a Double). Note that you would use the 'read' function to read integer values, too; digitToInt is just for dealing with single Chars, not entire integral values. The read function takes a String representation of anything that is "readable" (any type that is an instance of the Read type class) and converts it into a value of the requested type, so it works for Doubles, Ints, Integers, lists, and so on... Also, if I were you I'd use something other than "Num" as the name of the data constructor for numeric values in your Expr type. Num is already used for something else (it's the name of the general numeric type class), and although it's legal to have the same name for both, it might lead to confusion, especially when dealing with error messages. Good luck with learning Haskell and with your course! If you need more help there's also the #haskell IRC channel on freenode.nethttp://haskell.org/haskellwiki/IRC_channel, where you'll find lots of friendly people who would be glad to answer your questions. -Brent