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Fix #240: Document formatter behavior for negative hexadecimals #632

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Apr 6, 2024
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Fix #240: Document formatter behavior for negative hexadecimals #632

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22 changes: 22 additions & 0 deletions doc/semantics.md
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Expand Up @@ -52,6 +52,28 @@ As a consequence, the following apparent subtyping relationships hold:
Byte <:< Short <:< Int <:< Double
<:< Float <:<

#### Implications for formatting negative values in hexadecimal

Because there is no runtime difference between `Byte` `Short` and `Int`s (for sufficiently low values),
`java.util.Formatter` (and hence all formatting strings) assume `Int` to determine the padding when formatting negative hexadecimal values.

This leads to the following difference in format output:

{% highlight scala %}
val b: Byte = -38.toByte
"%x".format(b)
// JVM: "da"
// Scala.js: "ffffffda"
{% endhighlight %}

To achieve portable code, convert the value to an unsigned int first:

{% highlight scala %}
val b: Byte = -38.toByte
"%x".format(b & 0xff)
// "da" on both platforms
{% endhighlight %}

### `getClass()`

In Scala/JVM as well as Scala.js, when assigning a primitive value to an `Any` (or a generic type), and asking for its `getClass()`, Scala returns the *boxed class* of the value's type, rather than the primitive value.
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