Arbitrary precision reals?

[Fergus Henderson]

On 25-Mar-2003, Tom Pledger <Tom.Pledger@peace.com> wrote:
> 
> The floating point part of the GNU mp library looks difficult to fit
> into Haskell's numeric classes, because the type signatures in class
> Floating don't include a how-much-precision-do-you-want parameter.

How about using a function type which takes a precision and gives
you back an answer to that many digits, and making this function
type an instance of the numeric type classes?

E.g.

	data MPF_T	-- abstract, corresponds to GNU mp's mpf_t
	type PRECISION = Int
	type ARBITRARY_PRECISION_REAL = (PRECISION -> MPF_T)

	instance Floating ARBITRARY_PRECISION_REAL where
		sqrt x = (\prec -> mpf_sqrt_ui (x (prec + sqrt_lossage)))
		   where sqrt_lossage = 1  -- amount of precision lost by sqrt
		...

Here mpf_sqrt_ui would be an interface to GMP's mpf_sqrt_ui() function,
which takes as input an mpf_t and a precision and produces an mpf_t as output.

-- 
Fergus Henderson <fjh@cs.mu.oz.au>  |  "I have always known that the pursuit
The University of Melbourne         |  of excellence is a lethal habit"
WWW: <http://www.cs.mu.oz.au/~fjh>  |     -- the last words of T. S. Garp.