[Haskell-cafe] Type conversion problems

[Lauri Alanko]

On Sun, Jun 13, 2004 at 11:15:28AM +0200, Christian Hofer wrote:
> fac :: Integer -> Integer
> fac n = product [1..n]
> 
> term :: Double -> Integer -> Double
> term x n = (-1.0::Double)**(fromInteger n) * (x**(fromInteger (2*n + 
> 1))) /
> 	   (fromInteger (fac (2*n + 1)))

Why do you have all those type annotations? Simply writing directly:

fac n = product [1..n]
term x n = -1 ** n * (x ** (2 * n + 1)) / fac (2 * n + 1)

gives you functions for which are inferred the types (which you can of
course also give explicitly if you want):

fac :: (Enum a, Num a) => a -> a
term :: (Floating a, Enum a) => a -> a -> a

And the type variable a can then be instantiated for Double. If you
really need to take an Integer parameter, you only need a single
conversion:

term' x n = term x (fromIntegral n)

with the type:

term' :: (Floating a, Enum a, Integral b) => a -> b -> a

If all this looks confusing, then forget about polymorphism and just
give exact types in the type annotations:

fac :: Double -> Double
fac n = product [1..n]

term :: Double -> Double -> Double
term x n = -1 ** n * (x ** (2 * n + 1)) / fac (2 * n + 1)

term' :: Double -> Integer -> Double
term' x n = term x (fromIntegral n)

Hope this helps.


Lauri Alanko
la at iki.fi