Is Double FloatingPoint?

Using Swift
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1

Why doesn't this work? Is Double not FloatingPoint?

let val: Double = 1

func foo<T: FloatingPoint>() -> T {
    T(val) // Error: Cannot convert value of type 'Double' to expected argument type 'Int'
}
2

FloatingPoint has initializers from integers (docs), but no requirement that it can be initialized from another arbitrary FloatingPoint.

BinaryFloatingPoint does, however, have such an initializer: Apple Developer Documentation

1 Like
3

Interesting, thank you. What is the rationale for this behaviour that FloatingPoint can't be initialised from another FloatingPoint?

4

User-defined types can conform to FloatingPoint (or BinaryFloatingPoint, for that matter), but there’s no way that the standard library types could know how to initialize themselves from such an instance.

5

But that somehow possible with BinaryFloatingPoint, what's the difference here?

6

It can; it can use the protocol requirements.

If you take a look at what FloatingPoint provides, it's incredibly abstract. Some basic properties like isNaN, isFinite, basic maths operations and comparison, and the radix, exponent and significand. Importantly, the significand returns a value of type Self. So it's kind of hard to get the magnitude; you can calculate it, but it'll be the same type you started with, so it's not that helpful for type conversions.

BinaryFloatingPoint just has a lot more information, so you do a better job of interpreting which number its supposed to represent.

If you're looking for more nuanced design rationale, I don't know. "Reasons", I guess. But from a practical perspective looking at the protocols, I think it would be difficult/impossible to create a generic FloatingPoint-from-other-arbitary-FloatingPoint initializer. It's just not designed for that.

1 Like
7

According to @scanon, FloatingPoint also represents non-binary floating point formats. Though I’m not sure whether knowing a floating point format is binary makes it any easier to store its values using primitive stdlib types…

8

Correct, it is not possible to write an initializer that creates a floating-point value from a value of an arbitrary other floating-point type with the protocol’s available guarantees.

The initializer was originally a requirement of FloatingPoint and was revised to be moved down to BinaryFloatingPoint on account of its not being implementable with FloatingPoint APIs.

3 Likes
9

It's not impossible, but it would kinda suck, and we wouldn't want people to do it.

1 Like
10

Sure, for any particular concrete type I guess it’s possible but gross.

I was pretty certain when I wrote the default implementation that it wasn’t possible to write one for FloatingPoint-to-FloatingPoint conversion, but I also know that you have more insight on this :slight_smile: