Why isn’t it allowed to pass the constant `0` to a...
# announcements
m
Why isn’t it allowed to pass the constant
0
to a
Double
? Why
0.0
?
p
Because 0 is an int and not a double?
z
Unlike Java, kotlin doesn't convert number literals for you. This gives some of the reasoning.
m
@Paul Woitaschek no, it’s a number literal with no type. It only becomes an
Int
when inferring a type. @Zach Klippenstein (he/him) [MOD] actually Kotlin does.
p
Uh now that's interesting!
Where can I read up on this?
m
I don’t know but the link that @Zach Klippenstein (he/him) [MOD] mentioned is a good starting point. It’s actually mentioned there but for some reason it’s not available for floating point. See comment of first line.
👍 1
It would be great to have
0
as universal unitless value. In
kotlin-css
for example we could just write
top = 0
instead of
top = 0.px
. But that’s another more advanced use case 😄
r
universal unitless value
So,
null
m
No,
null
is absence of value.
0
means
0
. Whether the
div
is
0
cm
,
px
or
rem
away from the top doesn’t matter - it’s at the top. Hence it’s just
0
in CSS.
null
would mean that the distance to top is not defined.
r
I don't think there is a such thing as a universal unitless
0
. What
0
means is dependent on the unit. Usually systems line up their
0
values, but not always. For example,
0C != 0F
.
That may sound pedantic, but that is what universal means.
0
is used for a lot more than just CSS size definitions.
m
Once it has a type then yes. But a unitless
0
can be converted in any type that supports it. Just like we have in Kotlin already:
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val int: Int = 0
val long: Long = 0 // implies 0L

if (int == long) // not allowed
The same way I could write
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marginTop(0)
And allow that only for integer
0
and no other. The respective overload would resolve that properly and it only works if explicitly supported.
r
But, in your first example,
top = 0
still wouldn't work as
top
isn't a number.
And having a concept of letting types state whether they have support for the magic
0
sounds like it would cause far more problems than solve.
m
Yeah that’s why I don’t like
kotlin-css
and have written my own library. Properties are too inflexible, functions are much better here.
Zero
could work the same way as
Nothing?
does. It only has one valid value. Only difference is that it is inferred as
Int
and
Zero
is only used explicitly.
r
But how would that work for other types? For example,
top = Zero
. How would you be able to automatically convert
Zero
to a
LinearDimension
? That could get more complicated with, say, transforms. Should
Zero
mean
Identity
or a matrix full of `0`s?
m
I wouldn’t add a conversion feature unless Kotlin gets one in general. Thus I’d only support it for parameter overloads, just like
Nothing?
.
top(0)
With
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fun top(value: Zero) {
    top(LinearDimension("0")) // example
}
fun top(value: LinearDimension) { … }
0
is not a matrix and thus wouldn’t be an acceptable value. In that case you’d probably use
Matrix.identity
or alike.
r
That still seems like it would cause more problems than it solves to me, but I may just not be seeing the whole picture. That would also be a breaking change. For example, the following would fail:
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var x = 0
x = 5  // Error: Expected Zero, found Int
m
That’s why it should be explicit.
vax x = 0
//
Int
vax x: Zero = 0
//
Zero
Just like
var x = 0
//
Int
var x: Long = 0
//
Long
It doesn’t even have to be a real type. It could just be valid for overload resolution and parameters with that value are actually
Int
on the callee side - or totally inaccessible.
r
But
0
wouldn't be an
Int
, it would be a
Zero
. Adding special implicit cases like that is exactly the "magic" that Kotlin tries to avoid.
m
0
on its own has no type. Otherwise you couldn’t assign it to
Byte
,
Long
, etc.
Another path would be similar to what Swift does.
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class LinearDimension(val value: String) {
   companion object: ZeroConstructible {
      override fun constructFromZero() = LinearDimension("0")
   }
}

val foo: LinearDimension = 0
It’s an API that specifies how types can be constructed from literals. It’s not type coercing.
r
Hmm, I've never seen that before (I don't use Swift). Are there such constructs for any kind of literal, or is
0
special?
m
They don’t have it for
0
specifically. They have for
IntegerLiteral
,
FloatLiteral
,
StringLiteral
,
ListLiteral
(e.g. for Set construction) etc.
+
MapLiteral
r
In that case, you could achieve the same functionality (and more) by simply allowing setters of multiple types. For example
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var x: LinearDiminsion by ...
    set(value) { ... }
    set(value: Int) { set(value.px) }
m
That’s a potential explosion of overloads though
Zero
would be already but still an acceptable trade-off for simpler CSS API.
That would be incorrect
You can have unitless
0
but not unitless
1
in CSS.
r
Sure, but this would need to apply universally to Kotlin, not just CSS.
m
1
needs a unit and cannot be implicitly pixel.
Actually there are cases in CSS where you must remove the unit from
0
. For example in some
calc(…)
expressions. So
0
should never implicitly become
0px
.
r
That sounds like a problem with CSS, not an example of something Kotlin should try to emulate. 🙂
m
Why not? We could use
0
everywhere where it has the same meaning.
Int
,
Byte
,
Long
,
Double
and customs like
css.LinearDimension
.
We’re mixing up two things though. My initial question was about
0
for
Float
and
Double
😄
r
Indeed, but I was intentionally not discussing number literals. I'm talking about the magic
0
. That's where I came into the conversation (and prefixed my original comment accordingly to make that clear).
m
css.LinearDimension
is about literal
0
not the
Int
value
0
r
I don't really have anything to add on whether
0
should be a valid
Double
. I'm just adding my $0.02 on whether
0
should be a special unitless universal, and what that would entail.
m
It would be more like a special case of integer literal that can be specifically addressed.
A zero literal
r
Indeed. I'm still not convinced (yet?) that it's a good idea, but it's definitely given me something to thing about. Thanks for humoring me.
m
That’s okay. Doesn’t need to be solved by now :) It’s a topic for another day. And also more in the DSL space where it would be quite useful. Maybe in Math too, I don’t know. Back to using
0
literal for floating-point values.
👍 1
z
yall might be interested in the #language-proposals and #language-evolution channels if you’re not already in them