Question to the stdlib pros: why is there no exten...

# stdlib

[Olaf Gottschalk]

Question to the stdlib pros: why is there no extension function Number.toBigDecimal()? I struggle with this a lot as there are specific functions Int.toBigDecimal() and so on, even for the real tricky cases like Double.toBigDecimal() which does the "right" thing using the String constructor of BigDecimal rather than the double constructor which may lead to real spooky results...

So in my code, I write my own extension function:

fun Number.toBigDecimal(): BigDecimal = when(this) {
    is Short -> this.toBigDecimal()
    is Int -> this.toBigDecimal()
    .......
}

This is highly repetitive and error prone... so, why is this function just missing from the standard lib?

[Youssef Shoaib [MOD]]

Anyone can subclass Number and define their own type. There is no standard way to take an arbitrary user-defined Number class and find a BigDecimal that corresponds to it.

[Olaf Gottschalk]

Ok, I did not see Number as an open class anyone can use... yeah, makes sense. I just saw all of the toDouble(), toInt() functions defined inside Number and thought: where's toBigDecimal() - but I guess the answer to that simply is that BigDecimal is not a "platform class representing a number" as the KDoc states... I just use it so often cause it's on the JVM but maybe not on all platforms? If it were on all platforms, I would simply rephrase my question why this function is missing in the abstract Number class...

[Olaf Gottschalk]

It's a bummer this cannot be something like a sealed hierarchy were we are certain about the number of Number types...

[Youssef Shoaib [MOD]]

Adding toBigDecimal() as a required function in kotlin.Number would be conceivably possible (with a deprecation cycle and some warnings and such, and ofc a KMP BigDecimal type). The real issue is that java.lang.Number doesn't have such a method, and so there's java implementers of Number that we can do nothing about (note that kotlin.Number maps to java.lang.Number and vice versa)

[Olaf Gottschalk]

Didn't even think of Number coming from Java. So again, we suffer from Java's shortcomings. :( As BigDecimal literally is the only type that can cover all in terms of the contained value, it would be really sensible to have such a conversion function.

[Youssef Shoaib [MOD]]

Number is really badly designed anyways. It has no plus or other operators. It doesn't really have much sensible use. What would've been better is some interfaces like Addable, Multipliable, but that ship has sailed. With context parameters though, we can make something somewhat useful: https://github.com/kotlin-graphics/imgui/blob/master/core/src/main/kotlin/imgui/genericMath.kt. You could make something similar to convert numbers into BigDecimal easily, or, y'know, just stick with the extension method

[Klitos Kyriacou]

BigDecimal literally is the only type that can cover all in terms of the contained value

Not really. There are, for example rational numbers (with several third-party implementations out there). A BigDecimal cannot represent Rational(1, 3). On the other hand, rationals would be the only number type that could represent anything currently available in the stdlib (but not third-party libs).