Truncate should parse negative values too, or return error

[AlexandrosKyriakakis]

Truncate function would be useful to parse also negative values. So for example,

a := decimal.RequireFromString("5432")
b := a.Truncate(-2)

So b should return 5000, currently it returns a which is misleading. In my opinion it should either return 5000 or an error.

I don't see any reason having this check precision >= 0 here:

decimal/decimal.go

Lines 1755 to 1769 in 08afb35

	// Truncate truncates off digits from the number, without rounding.
	//
	// NOTE: precision is the last digit that will not be truncated (must be >= 0).
	//
	// Example:
	//
	// decimal.NewFromString("123.456").Truncate(2).String() // "123.45"
	func (d Decimal) Truncate(precision int32) Decimal {
	d.ensureInitialized()
	if precision >= 0 && -precision > d.exp {
	return d.rescale(-precision)
	}
	return d
	}

func (d Decimal) Truncate(precision int32) Decimal {
	d.ensureInitialized()
	if precision >= 0 && -precision > d.exp {
		return d.rescale(-precision)
	}
	return d
}

Since it would work, right out of the box.

Any thoughts or concerns?

[hiohiohio]

fyi. some of Round functions support negative value for places parameter.

decimal/decimal.go

Lines 1523 to 1524 in 08afb35

	// Round rounds the decimal to places decimal places.
	// If places < 0, it will round the integer part to the nearest 10^(-places).

[AlexandrosKyriakakis]

cc @mwoss