exact-real: Exact real arithmetic
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- exact-real-0.12.1.tar.gz [browse] (Cabal source package)
- Package description (as included in the package)
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| Versions [RSS] | 0.2.0.0, 0.2.1.0, 0.3.0.0, 0.4.0.0, 0.5.0.0, 0.7.1.0, 0.8.0.0, 0.8.0.2, 0.9.0, 0.10.0, 0.11.0, 0.11.1, 0.11.2, 0.11.3, 0.12.0, 0.12.1, 0.12.2, 0.12.3, 0.12.4, 0.12.4.1, 0.12.5, 0.12.5.1 |
|---|---|
| Dependencies | base (>=4.8 && <4.9), integer-gmp (<1.1.0.0), memoize (>=0.7 && <0.8), random (>=1.0 && <1.2) [details] |
| Tested with | ghc >=7.10 && <8 |
| License | MIT |
| Copyright | 2015 Joe Hermaszewski |
| Author | Joe Hermaszewski |
| Maintainer | Joe Hermaszewski <keep.it.real@monoid.al> |
| Category | Math |
| Home page | http://github.com/expipiplus1/exact-real |
| Bug tracker | http://github.com/expipiplus1/exact-real/issues |
| Source repo | head: git clone https://github.com/expipiplus1/exact-real |
| Uploaded | by jophish at 2016-03-31T18:02:28Z |
| Distributions | NixOS:0.12.5.1 |
| Reverse Dependencies | 2 direct, 0 indirect [details] |
| Downloads | 12786 total (36 in the last 30 days) |
| Rating | 2.25 (votes: 2) [estimated by Bayesian average] |
| Your Rating | |
| Status | Docs uploaded by user Build status unknown [no reports yet] |
Readme for exact-real-0.12.1
[back to package description]exact-real
Exact real arithmetic implemented by fast binary Cauchy sequences.
Motivating Example
Compare evaluating Euler's identity with a Float:
λ> let i = 0 :+ 1
λ> exp (i * pi) + 1 :: Complex Float
0.0 :+ (-8.742278e-8)
... and with a CReal:
λ> import Data.CReal
λ> let i = 0 :+ 1
λ> exp (i * pi) + 1 :: Complex (CReal 0)
0 :+ 0
Implementation
The basic operations have explanations and proofs of correctness here.
CReal's phantom type parameter n :: Nat represents the precision at which
values should be evaluated at when converting to a less precise representation.
For instance the definition of x == y in the instance for Eq evaluates x - y at precision n and compares the resulting Integer to zero. I think that
this is the most reasonable solution to the fact that lots of of operations
(such as equality) are not computable on the reals but we want to pretend that
they are for the sake of writing useful programs. Please see the
Caveats section for more information.
The CReal type is an instance of Num, Fractional, Floating, Real,
RealFrac, RealFloat, Eq, Ord, Show and Read. The only functions not
implemented are a handful from RealFloat which assume the number is
implemented with a mantissa and exponent.
There is a comprehensive test suite to test the properties of these classes.
The performance isn't terrible on most operations but it's obviously not nearly
as speedy as performing the operations on Float or Double. The only two
super slow functions are asinh and atanh at the moment.
Caveats
The implementation is not without its caveats however. The big gotcha is that
although internally the CReal ns are represented exactly, whenever a value is
extracted to another type such as a Rational or Float it is evaluated to
within 2^-p of the true value.
For example when using the CReal 0 type (numbers within 1 of the true value)
one can produce the following:
λ> 0.5 == (1 :: CReal 0)
True
λ> 0.5 * 2 == (1 :: CReal 0) * 2
False
Contributing
Contributions and bug reports are welcome!
Please feel free to contact me on GitHub or as "jophish" on freenode.
-Joe