[atombender]

This maps poorly to the mathematical concept of a closed interval [a, b], which can be written a ≤ x ≤ b for a set of x. An interval where a > b is usually a programming error.

To ensure only valid intervals are supported at the type system level, the function could perhaps be redefined as:

    function clamp(n: number, i: Interval<number>): number

Of course, you need to deal with the distinction between closed and open intervals. Clamping really only makes sense for closed ones.

[roenxi]

It maps very well onto the mathematical concept of a closed interval [a, b] where a and b are endpoints of the interval though. You're adding a constraint for no logical reason and it happens to be very hard to represent in a basic type system.

> An interval where a > b is usually a programming error.

If you want it to be, sure. Anything can be a programming error if the library author feels like it. We may as well put all sorts of constraints on clamp, it is probably an error if the caller uses a large number or a negative too. It is still bad design in a theoretical sense - the clamp function throws an error despite there being an obvious non-error return value. It isn't hard to meaningfully clamp 2 between 4 and 3.