[Haskell-cafe] type keeping rounding, typeable (and a difficulty)

isto isto.aho at dnainternet.net
Wed Nov 15 16:04:52 EST 2006


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)
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

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