[Haskell-cafe] Half-integer

[Thomas ten Cate]

On Sun, Jun 28, 2009 at 15:24, Andrew Coppin<andrewcoppin at btinternet.com> wrote:
> I just wrote a small module for dealing with half-integers. (That is, any
> number I/2 where I is an integer. Note that the set of integers is a subset
> of this; Wikipedia seems to reserve "half-integer" for such numbers that are
> *not* integers.)
>
>  module HalfInteger where
>
>  data HalfInteger i
>
>  instance (Eq i) => Eq (HalfInteger i)
>  instance (Ord i) => Ord (HalfInteger i)
>  instance (Integral i) => Show (HalfInteger i)
>  instance (Integral i) => Num (HalfInteger i)
>
>  half :: (Num i) => HalfInteger i
>
>  fromNum :: (Integral i, RealFrac x) => x -> HalfInteger i
>  toNum :: (Integral i, Fractional x) => HalfInteger i -> x
>
>  isInteger :: (Integral i) => HalfInteger i -> Bool
>
> Note carefully that the set of half-integers is *not* closed under
> multiplication! This means that for certain arguments, there are two
> reasonable products that could be returned. (E.g., 1/2 * 1/2 = 1/4, so 0 or
> 1/2 would be a reasonable rounding.) I haven't put a lot of effort into the
> rounding details of (*) or fromNum; which answer you get is kind of
> arbitrary. (However, addition and subtraction are exact, and for
> multiplications where an exact result is possible, you will get that
> result.)
>
> The Show instance outputs strings such as
>
>  fromInteger 5
>  fromInteger 5 + half
>  fromInteger (-5) - half
>
> depending on the isInteger predicate.
>
> Now, the question is... Is this useful enough to be worth putting on
> Hackage?

Out of curiosity, what are *you* using it for?

Thomas