On Wednesday, 19. December 2001 09:12, S.D.Mechveliani wrote:
[...]
> But if you still need Integer | Double, you can, for example,
> introduce a new type of a disjoint union of the above two, and then,
> to compute like this:
> pow (Intg 2) 2 --> Intg 4
> pow (Intg 2) (-2) --> D 0.25
> pow (D 2.0) (-2) --> D 0.25
> This is achieved by
>
> data PowerDom = Intg Integer | D Double deriving(Eq,Show)
>
> pow :: PowerDom -> Integer -> PowerDom
> pow x n = p x n
> where
> p (Intg m) n = if n > 0 then Intg $ powerInteger m n
> else D $ powerDouble (fromInteger m :: Double) n
> p(D d) n = D $ powerDouble d n
>
> powerInteger m n = m^n :: Integer
>
> powerDouble :: Double -> Integer -> Double
> powerDouble d n = ... usual way for float
>
> - something like this.
This seems to be what I want. I tried it this way:
module Main where
import System
main = do
[a1, a2] <- getArgs
let x = read a1
let y = read a2 in
putStrLn (show x ++ " ^ " ++ show y ++ " = " ++ show (pow x y))
data PowerNum = INT Integer | DBL Double deriving (Eq, Show, Read)
pow :: PowerNum -> Integer -> PowerNum
pow x y = z x y where
z (INT x) y = if y > 0 then
INT $ powInteger x y
else
DBL $ powDouble (fromInteger x) y
z (DBL x) y = DBL $ powDouble x y
powInteger x y
| x == 0 = 0
| y == 0 = 1
| y > 0 = x * powInteger x (y - 1)
powDouble x y
| x == 0 = 0
| y == 0 = 1
| y > 0 = 1 / x * powDouble x (y - 1)
While GHC compiled this code I get a runtime error:
Fail: Prelude.read: no parse
and HUGS reports:
ERROR: Illegal Haskell 98 class constraint in inferred type
*** Expression : pow 1 2
*** Type : Num PowerNum => PowerNum
Could you tell me what I did wrong? Thank you very much!
Toralf
>
>
> -----------------
> Serge Mechveliani
> [EMAIL PROTECTED]
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