Undocumented escape hatch

On my quest to a custom when-statement I did quite a bit of reading. The study of roast and Actions.nqp can lead to great gain in knowledge.

$ less -N S04-statements/given.t
136 # given returns the correct value:
137 {
138      sub ret_test($arg) {
139        given $arg {
140          when "a" { "A" }
141          when "b" { "B" }
142        }
143      }
144
145     is( ret_test("a"), "A", "given returns the correct value (1)" );
146     is( ret_test("b"), "B", "given returns the correct value (2)" );
147 }

As we can see in this example, the spec asks given to return the value provided to succeed. This is an ENODOC. We don’t have to depend on sink to turn the succeed value into a Routines return value.

my $result = do given 'a' {
    CONTROL { default { say 'seen ', .^name } }
    when Str { say 'Str'; succeed('It was a string.'); }
}
dd $result;
# OUTPUT: Str
          Str $result = "It was a string."

It’s a bit odd that we need the do thought, as given will always return at least Nil. The oddity doesn’t stop there. We can get hold of control exceptions. Some of which can return a value. That value is well hidden in nqp-land. Control-exceptions are clearly not an implementation details. So there is no reason for that limitation. Let’s remove it.

given 'a' {
    succeed .Str;
    CONTROL {
        when CX::Succeed {
            use nqp;
            my $vmex := nqp::getattr(nqp::decont($_), Exception, '$!ex');
            my $payload := nqp::getpayload($vmex);
            say 'seen succeed with payload: ', $payload;
        }
        default { say 'seen ', .^name; }
    }
}
# OUTPUT: seen succeed with payload: a

My expedition into nqp-land where started by the discovery, that CX::Succseed and CX::Proceed are swallowed by a hidden monster.

given 'a' {
    CONTROL { default { say 'seen ', .^name } }
    when Str { say 'Str'; succeed('It was a string.'); }
}
# OUTPUT:

$ less -N src/Perl6/Actions.nqp
9932     sub when_handler_helper($when_block) {
9933         unless nqp::existskey(%*HANDLERS, 'SUCCEED') {
9934             %*HANDLERS<SUCCEED> := QAST::Op.new(
9935                 :op('p6return'),
9936                 wrap_return_type_check(
9937                     QAST::Op.new(
9938                         :op('getpayload'),
9939                         QAST::Op.new( :op('exception') )
9940                     ),
9941                     $*DECLARAND) );

The first when or default clause add a fresh handler and only checks for SUCCEED and ignores any CONTROL blocks already present. Given that intercepting X::Control is specced, this is rather surprising.

Alas, adding exception handlers via macros, doesn’t work right now. This is not a pressing issue because macros are subject to change my RakuAST anyway and I might get the desired result with a Slang.