Examples from the reference don't work?

[osa1]

First time writing Scala. Trying the examples in https://docs.scala-lang.org/scala3/reference/experimental/cc.html, using Scala 3 built from source (commit 665f6d7).

In the section "By-Name Parameter Types" we have these two functions

def f1(x: => Int): Int =
  123

def f2(x: -> Int): Int =
  123

The text says the first one should allow captures but the second one doesn't. But this works:

class MyException extends Exception, Capability

def f1(x: => Int): Int =
  123

def f2(x: -> Int): Int =
  123

@main
def hello(): Unit =
  f1(if true then throw MyException() else 1)
  f2(if true then throw MyException() else 1)

Shouldn't the second call be rejected? Am I doing it wrong?

[odersky]

Exceptions are only tracked under the language import language.experimental.saferExceptions. So I think the reference needs to be updated. Here's a simpler example that shows purity checking works for by-name arguments:

import language.experimental.captureChecking

def f1(x: => Int): Int =
  123

def f2(x: -> Int): Int =
  123

def hello(c: Object^): Unit =
  f1({ println(c); 1 })
  f2({ println(c); 1 })

Compiling this gives

-- [E007] Type Mismatch Error: test.scala:14:5 ---------------------------------
14 |  f2({ println(c); 1 })
   |     ^^^^^^^^^^^^^^^^^
   |     Found:    () ?->{c} Int
   |     Required: () ?-> Int
   |
   | longer explanation available when compiling with `-explain`

[osa1]

Thanks @odersky for the answer.

I'm trying to understand how the capability-based tracking of exceptions scale as you combine functions that use capabilities.

I have this:

import caps.Capability
import java.io.FileOutputStream
import language.experimental.captureChecking
import language.experimental.saferExceptions

class E1 extends Exception, Capability
class E2 extends Exception, Capability
class E3 extends Exception, Capability
class E4 extends Exception, Capability

def f1(): Unit throws E1 =
  throw E1()

def f2(): Unit throws E2 =
  throw E2()

def f3(): Unit throws E3 =
  throw E3()

def f4(): Unit throws E4 =
  throw E4()

def f5(): Unit throws E3 | E4 =
  f3()
  f4()

// def f6(): Unit^{f5, f2, f1} =
def f6(): Unit throws E1 | E2 | E3 | E4 =
  f5()
  f2()
  f1()

Ideally I shouldn't have to list all of the exception types in f6. From the capture checking page I thought I could do something like:

def f6: Unit^{f5, f2, f1} =
  f5()
  f2()
  f1()

But this generates a few errors.. Is something like this allowed? What's the right syntax?

Thanks again.