[lionkor]

`bool metric` implies that `metric` can be YES or NO, that's clearer than what was there before.

Its a made-up example, this discussion is about style, not about semantics. To please those who would like to see a semantically improved version: (C++)

    enum class Unit {
        Meter,
        Inch,
        Foot,
        // ...
    };

    template<Unit U>
    struct UnitValue {
        double value;
    };

    template<Unit U>
    UnitValue<U> rectangle_area(UnitValue<U> side_a, UnitValue<U> side_b);


    // invocation example
    constexpr Unit unit = Unit::Meter;
    UnitValue<unit> result = rectangle_area<unit>({5.3}, {1.5});

    std::cout << result.value << std::endl;

There is still room for improvement, maybe some concepts to further generify it, some more comments, maybe a PDF with full documentation, ...