[Haskell-cafe] ANNOUNCE: approximate-equality 1.0 -- Newtype wrappers for approximate equality

Gregory Crosswhite gcross at phys.washington.edu
Tue Aug 3 00:32:12 EDT 2010

I am pleased to announce the release of the package "approximate-equality", which provides newtype wrappers that allow one to effectively override the equality operator of a value so that it is /approximate/ rather than /exact/.  The wrappers use type annotations to specify the tolerance; the 'Digits' type constructor has been provided for specifying the tolerance using type-level natural numbers.  Instances for all of the classes in the numerical hierarchy have been provided for the wrappers, so the wrapped values can mostly be used in the same way as the original values. (In fact, most of the time one doesn't even have to wrap the starting values, since expressions such as (1+sqrt 2/3) are automatically wrapped thanks to the 'fromIntegral' method of the 'Num' typeclass.)

The motivation behind this package is that there are times when working with floating point numbers that one would like the equality operator to check for approximate equality rather than exact equality.  For example, in one of my own projects I have a walker that  spits out a list of coordinate values that I keep track of, and since floating point arithmetic is inexact it will often arrive at the same point from two different paths and thus obtain slightly different values for the coordinates of that point.  I could have just written a function to do approximate matching of the point and be done with it, but then I can't leverage pre-built data structures such as Data.Set and Data.Map which use (==) for determining whether a key is a member.

This package is compatible with Haskell 2010, as it only uses the EmptyDataDecls extension, and the only package dependency is type-level-natural-numbers which itself is Haskell 2010.

Any feedback from the community is, of course, very welcome.


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