Apocalypse 4
by Larry Wall
|
Pages: 1, 2, 3, 4, 5, 6, 7, 8, 9
Editor's Note: this Apocalypse is out of date and remains here for historic reasons. See Synopsis 04 for the latest information.
Withdrawn RFCs
RFC 063: Exception handling syntax
RFC 113: Better constants and constant folding
Other decisions
C-style for loop
Due to syntactic ambiguities with the new for syntax of Perl 6,
the generalized C-style for loop is going to get its keyword changed
to loop. And for will now always mean "foreach". The expression
"pill" is now optional, so instead of writing an infinite loop like this:
for (;;) {
...
}
you can now write it like this:
loop {
...
}
C-style do {} while EXPR no longer supported
In Perl 5, when you used a while statement modifier on a statement
consisting of nothing but a do {}, something magical happened, and
the block would be evaluated once before the condition was evaluated.
This special-cased construct, seldom used and often misunderstood, will
no longer be in Perl 6, and in fact will produce a compile-time error
to prevent people from trying to use it. Where Perl 5 code has this:
do {
...
} while CONDITION;
Perl 6 code will use a construct in which the control flow is more explicit:
loop {
...
last unless CONDITION;
}
Bare blocks
In Perl 5, bare blocks (blocks used as statements) are once-through
loops. In Perl 6, blocks are closures. It would be possible to
automatically execute any closure in void context, but unfortunately,
when a closure is used as the final statement in an outer block,
it's ambiguous as to whether you wanted to return or execute the
closure. Therefore the use of a closure at the statement level
will be considered an error, whether or not it's in a void context.
Use do {} for a "once" block, and an explicit return or sub
when you want to return a reference to the closure.
continue block
The continue block changes its name to NEXT and moves inside the
block it modifies, to work like POST blocks. Among other things, this
allows NEXT blocks to refer to lexical variables declared within the
loop, provided the NEXT block is place after them. The generalized
loop:
loop (EXPR1; EXPR2; EXPR3) { ... }
can now be defined as equivalent to:
EXPR1;
while EXPR2 {
NEXT { EXPR3 }
...
}
(except that any variable declared in EXPR3 would have different
lexical scope). The NEXT block is called only before attempting
the next iteration of the loop. It is not called when the loop is
done and about to exit. Use a POST for that.
Well, that about wraps it up for now. You might be interesting to know that I'm posting this from the second sesquiannual Perl Whirl cruise, on board the Veendam, somewhere in the Carribean. If the ship disappears in the Bermuda Triangle, you won't have to worry about the upcoming Exegesis, since Damian is also board. But for now, Perl 6 is cruising along, the weather's wonderful, wish you were here.
