IntegerInterval: add memberCount

[ncfavier]

Returns the number of integers that lie within an integer interval.

This is different from the width and cannot simply be computed as (+ 1) . width because of the null interval.

Addresses #39

EDIT: I initially named this magnitude because I misunderstood what Numeric.Interval.magnitude does. Feel free to suggest a better name.

[Bodigrim]

Shall we make it total, returning Maybe Integer?

[ncfavier]

In that case we might as well return Extended Integer, so that the memberCount of an unbounded interval is PosInf?

[Bodigrim]

Imagine yourself on the consuming end, where you have to pattern match on the output of memberCount. What would you do for NegInf? Throw error "this should never happen"? I'd prefer to receive Maybe.

[Bodigrim]

As usual, the shorter PR the longer bikeshedding :)

I wonder if Maybe Natural would be a better return type. Two benefits:

• It's the smallest possible set of values, making memberCount surjective.
• It's more distinguishing, e. g., one cannot replace memberCount with pickup.
  An obvious penalty:
• Users are likely to need more fromIntegral / toInteger.

I'm split. @ncfavier what do you think? Let's do what you prefer.

[ncfavier]

If this were Agda I would definitely insist on returning Maybe ℕ, but in a low-level language like Haskell I don't think it's worth the trouble. You don't pattern match on Integer.

[Bodigrim]

Thanks!

[ncfavier]

Thanks! A release including this change (and my last one from last year) would be appreciated.

[Bodigrim]

Done: https://hackage.haskell.org/package/data-interval-2.1.2