type keeping rounding, typeable (and a difficulty)
Hi, I've been trying to compile the following function (rounding to a desired degree): roundDec :: (Num a, Typeable a) => Int -> a -> a roundDec d a = let t = show (typeOf a) in case t of "Double" -> roundDDec d a "Complex Double" -> roundCDec d a otherwise -> a -- or something The two other functions are roundCDec :: (RealFloat a) => Int -> Complex a -> Complex a roundCDec d (c :+ b) = (roundDDec d c :+ roundDDec d b) and roundDDec :: (RealFloat a) => Int -> a -> a roundDDec d a = a -- or somegthing Compiler gives the following error message: Couldn't match expected type `Complex a' against inferred type `a1' (a rigid variable) `a1' is bound by the type signature for `roundDec' at FFT.hs:57:17 In the second argument of `roundCDec', namely `a' In the expression: roundCDec d a In a case alternative: "Complex Double" -> roundCDec d a If in the roundDDec a's are replaced with Double, there will be similar error message from the "Double"-line. The functionality can be written differently, but I wanted to try write rounding having in a signature at least "(Num a) => Int -> a -> a". Again, any help would be appreciated a lot! Thanks in advance! br, Isto
isto wrote: ] I've been trying to compile the following function ] (rounding to a desired degree): ] ] roundDec :: (Num a, Typeable a) => Int -> a -> a ] roundDec d a = ] let t = show (typeOf a) ] in case t of ] "Double" -> roundDDec d a ] "Complex Double" -> roundCDec d a ] otherwise -> a -- or something ] ] The two other functions are ] ] roundCDec :: (RealFloat a) => Int -> Complex a -> Complex a ] roundCDec d (c :+ b) = (roundDDec d c :+ roundDDec d b) ] and ] roundDDec :: (RealFloat a) => Int -> a -> a ] roundDDec d a = a -- or somegthing Maybe you want type classes instead?
import Complex
class Round a where roundD :: Int -> a -> a
instance Round Double where roundD d a = a
instance (Round a, RealFloat a) => Round (Complex a) where roundD d (c :+ b) = (roundD d c :+ roundD d b)
ke, 2006-11-15 kello 13:31 -0800, Greg Buchholz kirjoitti:
isto wrote: ] let t = show (typeOf a) ] in case t of ] "Double" -> roundDDec d a ] "Complex Double" -> roundCDec d a
Maybe you want type classes instead?
aaaa yes, I was blind... Thanks! I'll guess the reason it didn't compile was different types at case branches (am I wrong?) Anyhow, do you know that is it possible to choose the return type somehow in the spirit above? br, Isto
isto wrote: ] > isto wrote: ] > ] let t = show (typeOf a) ] > ] in case t of ] > ] "Double" -> roundDDec d a ] > ] "Complex Double" -> roundCDec d a ] ] I'll guess the reason it didn't compile was different ] types at case branches (am I wrong?) Correct. ] Anyhow, do you know that is it possible to choose the return type ] somehow in the spirit above? Maybe you want something like...
roundDec d (Left a) = Left (roundDDec d a) roundDec d (Right a) = Right (roundCDec d a)
roundCDec :: (RealFloat a) => Int -> Complex a -> Complex a roundCDec d (c :+ b) = (roundDDec d c :+ roundDDec d b)
roundDDec :: (RealFloat a) => Int -> a -> a roundDDec d a = a -- or somegthing
Greg Buchholz
Hi & thanks! to, 2006-11-16 kello 14:02 -0800, Greg Buchholz kirjoitti:
] I'll guess the reason it didn't compile was different ] types at case branches (am I wrong?)
Correct.
] Anyhow, do you know that is it possible to choose the return type ] somehow in the spirit above?
Maybe you want something like...
This time, however, I'm not sure after seeing oleg's email: http://www.haskell.org/pipermail/haskell/2006-November/018736.html I'll have yet to re-read it carefully to be sure... :) br, Isto
On Thu, Nov 16, 2006 at 10:44:59PM +0200, isto wrote:
I'll guess the reason it didn't compile was different types at case branches (am I wrong?) Anyhow, do you know that is it possible to choose the return type somehow in the spirit above?
GADTs let you do this. And they even omit the run time type check. though, the type class solution is the correct way to do this sort of thing. data Value a where IntLike :: Int -> Value Int CharLike :: Char -> Value Char f :: Value a -> a f x = case x of IntLike x -> x + 1 CharLike x -> x:" plus one" there is also Data.Dynamic and existential types which are related to the task. John -- John Meacham - ⑆repetae.net⑆john⑈
participants (3)
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Greg Buchholz -
isto -
John Meacham