Uncertain<T>

Arguably Uncertain should be the default, and you should have to annotate a type as certain T when you are really certain. ;)

Only for physical measurements. For things like money, you should be pretty certain, often down to exact fractional cents.

It appears that a similar approach is implemented in some modern Fortran libraries.

A person might have mistyped a price, a barcode may have been misread, the unit prices might be correct but the quantity could be mistaken. Modeling uncertainty well isn't just about measurement error from sensors.

I wonder what it'd look like to propagate this kind of uncertainty around. You might want to check the user's input against a representative distribution to see if it's unusual and, depending on the cost of an error vs the friction of asking, double-check the input.