=head1 TITLE

Exegesis 4: Syntax

=head1 AUTHOR

Damian Conway <damian@conway.org>

=head1 VERSION

  Maintainer: Larry Wall <larry@wall.org>
  Date: 2 Apr 2002
  Last Modified: 30 May 2006
  Number: 4
  Version: 2

[Update: Please note that this was written several years ago, and
a number of things have changed since then.  Rather than changing
the original document, we'll be inserting "Update" notes like this
one to tell you where the design has since evolved.  (For the better,
we hope).  In any event, for the latest Perl 6 design (or to figure out
any cryptic remarks below) you should read the Synopses, which are kept
very much more up-to-date than either the Apocalypses or Exegeses.]

=over

=item B<I<And I'd se-ell my-y so-oul for flow of con-tro-ol ... over Perl>>

=over

=item B<-- The Motels, "Total Control" (Perl 6 remix)>

=back

=back

In Apocalypse 4, Larry explains the fundamental changes to flow and
block control in Perl 6. The changes bring fully integrated exceptions;
a powerful new switch statement; a coherent mechanism for polymorphic
matching; a greatly enhanced C<for> loop; and unification of blocks,
subroutines and closures.

Let's dive right in.

=head1 "Now, Witness the Power of This Fully I<Operational> Control
Structure"

We'll consider a simple interactive 
L<RPN calculator|"http://www.calculator.org/rpn.html">. The real
thing would have many more operators and values, but that's not
important right now.

    class Err::BadData is Exception {...}

    module Calc;

[Update: This syntactic form is allowed only for the entire file.]

    my class NoData is Exception {
        method warn(*@args) { die @args }
    }

    my %var;

    my sub get_data ($data) {
        given $data {
            when /^\d+$/    { return %var{""} = $_ }
            when 'previous' { return %var{""} // fail NoData }
            when %var       { return %var{""} = %var{$_} }
            default         { die Err::BadData : msg=>"Don't understand $_" }
        }  
    }

    sub calc (str $expr, int $i) {
        our %operator is private //= (
            '*'  => { $^a * $^b },
            '/'  => { $^a / $^b },
            '~'  => { ($^a + $^b) / 2 },
        );

[Update: There is no private property.  And this would be a good place
for a constant declaration.

        constant %operator =
            '*'  => { $^a * $^b },
            '/'  => { $^a / $^b },
            '~'  => { ($^a + $^b) / 2 };

Note also that the right side of a list= doesn't require parens any more.]

        my @stack;
        my $toknum = 1;
        for split /\s+/, $expr -> $token {
            try {
                when %operator {
                    my @args = splice @stack, -2;
                    push @stack, %operator{$token}(*@args)
                }
                when '.', ';', '=' {
                    last
                }

                use fatal;
                push @stack, get_data($token);

                CATCH {
                    when Err::Reportable     { warn $!; continue }
                    when Err::BadData        { $!.fail(at=>$toknum) }
                    when NoData              { push @stack, 0 }
                    when /division by zero/  { push @stack, Inf }
                }
            }

            NEXT { $toknum++ }
        }
        fail Err::BadData: msg=>"Too many operands" if @stack > 1;
        return %var{'$' _ $i} = pop(@stack) but true;

[Update: Concatenation is now C<~> instead of underline.]

    }

    module main;

    for 1..Inf -> $i {
        print "$i> ";
        my $expr = <> err last;  

[Update: C<< <> >> is now C<=*> or some such.]

        print "$i> $( Calc::calc(i=>$i, expr=>$expr) )\n";
    }

=head1 An Exceptionally Promising Beginning

The calculator is going to handle internal and external errors using
Perl 6's OO exception mechanism. This means that we're going to need
some classes for those OO exceptions to belong to.

To create those classes, the C<class> keyword is used. For example:

    class Err::BadData is Exception {...}

After this declaration, C<Err::BadData> is a class name (or rather, by
analogy to "filehandle," it's a "classname"). Either way, it can then
be used as a type specifier wherever Perl 6 expects one. Unlike Perl 5,
that classname is not a bareword string: It's a genuine first-class
symbol in the program. In object-oriented terms, we could think of a
classname as a meta-object -- an object that describes the attributes
and behavior of other objects.

Modules and packages are also first class in Perl 6, so we can also
refer to their names directly, or take references to them, or look them
up in the appropriate symbol table.

Classes can take properties, just like variables and values. Generally,
those properties will specify variations in the behavior of the class.
For example:

    class B::Like::Me is interface;

specifies that the C<B::Like::Me> class defines a (Java-like) interface
that any subclass must implement.

[Update: We now use C<role> declarations for interfaces, by the way.]

The C<is Exception> is not, however, a standard property. Indeed,
C<Exception> is the name of another (standard, built-in) class. When a
classname like this is used as if it were a property, the property it
confers is inheritance. Specifically, C<Err::BadData> is defined as
inheriting from the C<Exception> base class. In Perl 5, that would have
been:

    # Perl 5 code
    package Err::BadData;
    use base 'Exception';

So now class C<Err::BadData> will have all the exceptionally useful
properties of the C<Exception> class.

Having classnames as "first class" symbols of the program means that
it's also important to be able to pre-declare them (to avoid
compile-time "no such class or module" errors). So we need a new syntax
for declaring the existence of classes/modules/packages, without
actually defining their behavior.

To do that we write:

    class MyClass {...}

That right. That's real, executable, Perl 6 code.

We're defining the class, but using the new Perl 6 "yada-yada-yada"
operator in a block immediately after the classname. By using the
"I'm-eventually-going-to-put-something-here-but-not-just-yet" marker,
we indicate that this definition is only a stub or placeholder. In this
way, we introduce the classname into the current scope without needing
to provide the complete description of the class.

By the way, this is also the way we can declare other types of symbols
in Perl 6 without actually defining them:

    module Alpha {...}
    package Beta {...}
    method Gamma::delta(Gamma $self: $d1, $d2) {...}
    sub epsilon() {...}

In our example, the C<Err::BadData> classname is introduced in
precisely that way:

    class Err::BadData is Exception {...}

which means that we can refer to the class by name, even though it has
not yet been completely defined.

In fact, in this example, C<Err::BadData> is I<never> completely
defined. So we'd get a fatal compile-time error: "Missing definition
for class Err::BadData." Then we'd realize we either forgot to
eventually define the class, or that we had really meant to write:

    class Err::BadData is Exception {}   # Define new exception class with
                                         # no methods or attributes
                                         # except those it inherits
                                         # See below.

=head1 Lexical Exceptions

Most of the implementation of the calculator is contained in the
C<Calc> module. In Perl 6, modules are specified using the C<module>
keyword:

    module Calc;

which is similar in effect to a Perl 5:

    # Perl 5 code
    package Calc;

Modules are not quite the same as packages in Perl 6. Most
significantly, they have a different export mechanism: They export via
a new, built-in, declarative mechanism (which will be described in a
future Apocalypse) and the symbols they export are exported lexically
by default.

The first thing to appear in the module is a class declaration:

    my class NoData is Exception {
        method warn(*@args) { die @args }
    }

This is another class derived from C<Exception>, but one that has two
significant differences from the declaration of C<class Err::BadData>:

=over

=item *

The leading C<my> makes it lexical in scope, and

=item *

the trailing braces give it an associated block in which its
attributes and methods can be specified.

=back

Let's look at each of those.

C<NoData> exceptions are only going to be used within the C<Calc>
module itself. So it's good software engineering to make them visible
only within the module itself.

Why? Because if we ever attempt to refer to the exception class outside
C<Calc> (e.g. if we tried to catch such an exception in C<main>), then
we'll get a compile-time "No such class: NoData" error. Any such errors
would indicate a flaw in our class design or implementation.

In Perl 6, classes are first-class constructs. That is, like variables
and subroutines, they are "tangible" components of a program, denizens
of a symbol table, able to be referred to both symbolically and by
explicit reference:

    $class = \Some::Previously::Defined::Class;

    # and later

    $obj = $class.new();

Note that the back slash is actually optional in that first line, just
as it would be for an array or hash in the same position.

"First class" also means that classnames live in a symbol table. So it
follows that they can be defined to live in the current I<lexical>
symbol table (i.e. C<%MY::>), by placing a C<my> before them.

A lexical class or module is only accessible in the lexical scope in
which it's declared. Of course, like Perl 5 packages, Perl 6 classes
and modules don't usually I<have> an explicit lexical scope associated
with their declaration. They are implicitly associated with the
surrounding lexical scope (which is normally a file scope).

But we can give them their own lexical scope to preside over by adding
a block at the end of their declaration:

    class Whatever {
        # definition here
    }

This turns out to be important. Without the ability to specify a
lexical scope over which the class has effect, we would be stuck with
no way to embed a "nested" lexical class:

    class Outer;
    # class Outer's namespace

    my class Inner;

    # From this line to the end of the file 
    # is now in class Inner's namespace

In Perl 6, we avoid this problem by writing:

    class Outer;
    # class Outer's namespace

    my class Inner {
        # class Inner's namespace
    }

    # class Outer's namespace again

In our example, we use this new feature to redefine C<NoData>'s C<warn>
method (upgrading it to a call to C<die>). Of course, we could also
have done that with just:

    my class NoData is Exception;       # Open NoData's namespace
    method warn(*@args) { die @args }   # Defined in NoData's namespace

but then we would have needed to "reopen" the C<Calc> module's
namespace afterward:

    module Calc;                        # Open Calc's namespace

    my class NoData is Exception;       # Open NoData's (nested) namespace
    method warn(*@args) { die @args }   # Defined in NoData's namespace

    module Calc;                        # Back to Calc's namespace

[Update: And, in fact, that package-switching syntax is now disallowed.
You have to use the block form for any declaration but the file scope.]

Being able to "nest" the C<NoData> namespace:

    module Calc;                            # Open Calc's namespace

    my class NoData is Exception {          # Open NoData's (nested) namespace
        method warn(*@args) { die @args }   # Defined in NoData's namespace
    }

    # The rest of module Calc defined here.

is much cleaner.

By the way, because classes can now have an associated block, they can
even be anonymous:

    $anon_class = class { 
        # definition here
    };

    # and later

    $obj = $anon_class.new();

which is a handy way of implementing "singleton" objects:

    my $allocator = class { 
                        my $.count = "ID_000001";
                        method next_ID { $.count++ }
                    }.new;

    # and later...

    for @objects {
        $_.set_id( $allocator.next_ID );
    }

=head1 Maintaining Your State

To store the values of any variables used by the calculator, we'll use
a single hash, with each key being a variable name:

    my %var;

Nothing more to see here. Let's move along.

=head1 It's a Given

The C<get_data> subroutine may be given a number (i.e. a literal
value), a numerical variable name (i.e. C<'$1'>, C<'$2'>, etc.) , or
the keyword C<'previous'>.

It then looks up the information in the C<%var> hash, using a switch
statement to determine the appropriate look-up:

    my sub get_data ($data) {
        given $data {

The C<given $data> evaluates its first argument (in this case,
C<$data>) in a scalar context, and makes the result the "topic" of each
subsequent C<when> inside the block associated with the C<given>.
(Though, just between us, that block is merely an anonymous closure
acting as the C<given>'s second argument -- in Perl 6 I<all> blocks are
merely closures that are slumming it.)

Note that the C<given $data> statement also makes C<$_> an alias for
C<$data>. So, for example, if the C<when> specifies a pattern:

    when /^\d+$/  { return %var{""} = $_ }

then that pattern is matched against the contents of C<$data> (i.e.
against the current topic). Likewise, caching and returning C<$_> when
the pattern matches is the same as caching and returning C<$data>.

After a C<when>'s block has been selected and executed, control
automatically passes to the end of the surrounding C<given> (or, more
generally, to the end of whatever block provided the C<when>'s topic).
That means that C<when> blocks don't "fall through" in the way that
C<case> statements do in C.

You can also explicitly send control to the end of a C<when>'s
surrounding C<given>, using a C<break> statement. For example:

    given $number {
        when /[02468]$/ {
            if ($_ == 2) {
                warn "$_ is even and prime\n";
                break;
            }           
            warn "$_ is even and composite\n";
        }
        when &is_prime {
            warn "$_ is odd and prime\n";
        }
        warn "$_ is odd and composite\n";
    }

Alternatively, you can explicitly tell Perl not to automatically
C<break> at the end of the C<when> block. That is, tell it to "fall
through" to the statement immediately after the C<when>. That's done
with a C<continue> statement (which is the new name for The Statement
Formerly Known As C<skip>):

    given $number {
        when &is_prime   { warn "$_ is prime\n"; continue; }
        when /[13579]$/  { warn "$_ is odd"; }
        when /[02468]$/  { warn "$_ is even"; }
    }

In Perl 6, a C<continue> means: "continue executing from the next
statement after the current C<when>, rather than jumping out of the
surrounding C<given>." It has nothing to do with the old Perl 5
C<continue> block, which in Perl 6 becomes C<NEXT>.

The "topic" that C<given> creates can also be aliased to a name of our
own choosing (though it's I<always> aliased to C<$_> no matter what
else we may do). To give the topic a more meaningful name, we just need
to use the "topical arrow":

    given check_online().{active}{names}[0] -> $name {
        when /^\w+$/  { print "$name's on first\n" }
        when /\?\?\?/    { print "Who's on first\n" }
    }

[Update: Would now be written more like:

    given check_online()<active><names>[0] -> $name {
        when /^ \w+ $/   { say "$name's on first" }
        when / \?\?\? /  { say "Who's on first" }
    }

]

Having been replaced by the dot, the old Perl 5 arrow operator is given
a new role in Perl 6. When placed after the topic specifier of a
control structure (i.e. the scalar argument of a C<given>, or the list
of a C<for>), it allows us to give an extra name (apart from C<$_>) to
the topic associated with that control structure.

In the above version, the C<given> statement declares a lexical
variable C<$name> and makes it yet another way of referring to the
current topic. That is, it aliases both C<$name> and C<$_> to the value
specified by C<check_online().{active}{names}[0]>.

This is a fundamental change from Perl 5, where C<$_> was only aliased
to the current topic in a C<for> loop. In Perl 6, the current topic --
whatever its name and however you make it the topic -- is I<always>
aliased to C<$_>.

That implies that everywhere that Perl 5 used C<$_> as a default (i.e.
C<print>, C<chomp>, C<split>, C<length>, C<eval>, etc.), Perl 6 uses
the current topic:

    for @list -> $next {        # iterate @list, aliasing each element to 
                                # $next (and to $_)
        print if length > 10;   # same as: print $next if length $next > 10
        %count{$next}++;
    }

[Update: There is no C<length> function any more.  You have to specify
C<.chars> or C<.bytes> or some such.]

This is subtly different from the "equivalent" Perl 5 code:

    # Perl 5 code
    for my $next (@list) {      # iterate @list, aliasing each element to 
                                # $next (but not to $_)
        print if length > 10;   # same as: print $_ if length $_ > 10
                                # using the $_ value from *outside* the loop
        %count{$next}++;
    }

If you had wanted this Perl 5 behavior in Perl 6, then you'd have to
say explicitly what you meant:

    my $outer_underscore := $_;
    for @list -> $next {
        print $outer_underscore
            if length $outer_underscore > 10;
        %count{$next}++;
    }

which is probably a good thing in code that subtle.

Oh, and yes: the C<p52p6> translator program I<will> take that new
behavior into account and correctly convert something pathological
like:

    # Perl 5 code
    while (<>) {
        for my $elem (@list) {
            print if $elem % 2;
        }
    }

to:

    # Perl 6 code
    for <> {
        my $some_magic_temporary_variable := $_;
        for @list -> $elem {
            print $some_magic_temporary_variable if $elem % 2;
        }
    }

Note that this works because, in Perl 6, a call to C<< <> >> is
lazily evaluated in list contexts, including the list of a C<for> loop.

[Update: The first argument to a "pointy sub" is always aliased to C<$_>
now as well.]

=head1 Other whens

The remaining cases of the data look-up are handled by subsequent
C<when> statements. The first:

    when 'previous' { return %var{""} // fail NoData }

handles the special keyword C<"previous">. The previous value is always
stored in the element of C<%var> whose key is the empty string.

If, however, that previous value is undefined, then the defaulting
operator -- C<//> -- causes the right-hand side of the expression to be
evaluated instead. That right-hand side is a call to the C<fail> method
of class C<NoData> (and could equally have been written
C<NoData.fail()>).

The standard C<fail> method inherited from the C<Exception> class
constructs an instance of the appropriate class (i.e. an exception
object) and then either throws that exception (if the C<use fatal>
pragma is in effect) or else returns an C<undef> value from the scope
in which the C<fail> was invoked. That is, the C<fail> acts like a
C<die SomeExceptionClass> or a C<return undef>, depending on the state
of the C<use fatal> pragma.

[Update: Instead of returning a bare C<undef>, the current language
returns a kind of undef that contains the unthrown exception that would
have been thrown under C<use fatal>.]

This is possible because, in Perl 6, I<all> flow-of-control --
including the normal subroutine C<return> -- is exception-based. So,
when it is supposed to act like a C<return>, the C<Exception::fail>
method simply throws the special C<Ctl::Return> exception, which
C<get_data>'s caller will (automagically) catch and treat as a normal
return.

So then why not just write the usual:

    return undef;

instead?

The advantage of using C<fail> is that it allows the I<callers> of
C<get_data> to decide how that subroutine should signal failure. As
explained above, normally C<fail> fails by returning C<undef>. But if a
C<use fatal> pragma is in effect, any invocation of C<fail> instead
throws the corresponding exception.

What's the advantage in that? Well, some people feel that certain types
of failures ought to be taken deadly seriously (i.e. they should kill
you unless you explicitly catch and handle them). Others feel that the
same errors really aren't all that serious and you should be allowed
to, like, chill man and just groove with the heavy consequences, dude.

The C<fail> method allows you, the coder, to stay well out of that kind
of fruitless religious debate.

When you use C<fail> to signal failure, not only is the code nicely
documented at that point, but the mode of failure becomes
caller-selectable. Fanatics can C<use fatal> and make each failure
punishable by death; hippies can say C<no fatal> and make each failure
just return C<undef>.

[Update: The default is now somewhere between those extremes; to throw
an explicit exception if the unthrown exception is not examined in
some fashion before being thrown away.]

You no longer have to get caught up in endless debate as to whether the
exception-catching:

    try { $data = get_data($str) }
        // warn "Couldn't get data" }

is inherently better or worse than the C<undef>-sensing:

    do { $data = get_data($str) }
        // warn "Couldn't get data";

Instead, you can just write C<get_data> such that There's More Than One
Way To Fail It.

By the way, C<fail> can fail in other ways, too: in different contexts
or under different pragmas. The most obvious example would be inside a
regex, where it would initiate back-tracking. More on that in
Apocalypse 5.

=head1 Still Other Whens

Meanwhile, if C<$data> isn't a number or the C<"previous"> keyword,
then maybe it's the name of one of the calculator's variables. The
third C<when> statement of the switch tests for that:

    when %var   { return %var{""} = %var{$_} }

If a C<when> is given a hash, then it uses the current topic as a key
in the hash and looks up the corresponding entry. If that value is
true, then it executes its block. In this case, that block caches the
value that was looked up (i.e. C<%var{$_}>) in the "previous" slot and
returns it.

"Aha!" you say, "that's a bug! What if the value of C<%var{$_}> is
false?!" Well, if it were possible for that to ever happen, then it
certainly I<would> be a bug, and we'd have to write something ugly:

    when defined %var{$_}   { return %var{""} = %var{$_} }

But, of course, it's much easier just to redefine Truth, so that any
literal zero value stored in C<%var> is no longer false. See below.

Finally, if the C<$data> isn't a literal, then a C<"previous">, or a
variable name, it must be an invalid token, so the default alternative
in the switch statement throws an C<Err::BadData> exception:

    default     { die Err::BadData : msg=>"Don't understand $_" }

Note that, here again, we are actually executing a method call to:

    Err::BadData.die(msg=>"Don't understand $_");

as indicated by the use of the colon after the classname.

Of course, by using C<die> instead of C<fail> here, we're giving
clients of the C<get_data> subroutine no choice but to deal with
C<Err::BadData> exceptions.

=head1 An Aside: the "Smart Match" Operator

The rules governing how the argument of a C<when> is matched against
the current topic are designed to be as DWIMish as possible. Which
means that they are actually quite complex. They're listed in
Apocalypse 4, so we won't review them here.

Collectively, the rules are designed to provide a generic "best attempt
at matching" behavior. That is, given two values (the current topic and
the C<when>'s first argument), they try to determine whether those
values can be combined to produce a "smart match" -- for some
reasonable definitions of "smart" and "match."

That means that one possible use of a Perl 6 switch statement is simply
to test I<whether> two values match without worrying about I<how> those
two values match:

    sub hey_just_see_if_dey_match_willya ($val1, $val2) {
        given $val1 {
            when $val2 { return 1 }
            default    { return 0 }
        }
    }

That behavior is sufficiently useful that Larry wanted to make it much
easier to use. Specifically, he wanted to provide a generic "smart
match" operator.

So he did. It's called C<=~>.

[Update: Now called C<~~> instead.]

Yes, the humble Perl 5 "match a string against a regex" operator is
promoted in Perl 6 to a "smart-match an I<anything> against an
I<anything>" operator. So now:

    if ($val1 =~ $val2) {...}

works out the most appropriate way to compare its two scalar operands.
The result might be a numeric comparison (C<$val1 == $val2>) or a
string comparison (C<$val1 eq $val2>) or a subroutine call
(C<$val1.($val2)>) or a pattern match (C<$val1 =~ /$val2/>) or whatever
else makes the most sense for the actual run-time types of the two
operands.

This new turbo-charged "smart match" operator will also work on arrays,
hashes and lists:

    if @array =~ $elem {...}        # true if @array contains $elem

    if $key =~ %hash {...}          # true if %hash{$key}

    if $value =~ (1..10) {...}      # true if $value is in the list

    if $value =~ ('a',/\s/,7) {...} # true if $value is eq to 'a'
                                    #   or if $value contains whitespace
                                    #   or if $value is == to 7

[Update: lists are no longer automatically smart matched distributively.
You can always use C<any(...)> for that, or the C<|> junctional operator.]

That final example illustrates some of the extra intelligence that Perl
6's C<=~> has: When one of its arguments is a list (I<not> an array),
the "smart match" operator recursively "smart matches" each element and
ORs the results together, short-circuiting if possible.

=head1 Being Calculating

The next component of the program is the subroutine that computes the
actual results of each expression that the user enters. It takes a
string to be evaluated and an integer indicating the current iteration
number of the main input loop (for debugging purposes):

    sub calc (str $expr, int $count) {

=head1 Give us a little privacy, please

Perl 5 has a really ugly idiom for creating "durable" lexical
variables: variables that are lexically scoped but stick around from
call to call.

If you write:

    sub whatever {
        my $count if 0;
        $count++;
        print "whatever called $count times\n";
    }

then the compile-time aspect of a C<my $count> declaration causes
C<$count> to be declared as a lexical in the subroutine block. However,
at run-time -- when the variable would normally be (re-)allocated --
the C<if 0> prevents that process. So the original lexical variable is
not replaced on each invocation, and is instead shared by them all.

This awful C<if 0> idiom works under most versions of Perl 5, but it's
really just a freakish accident of Perl's evolution, not a carefully
designed and lovingly crafted feature. So just say "No!".

Perl 6 allows us to do the same thing, but without feeling the need to
wash afterward.

To understand how Perl 6 cleans up this idiom, notice that the durable
variable is really much more; like a package variable that just happens
to be accessible only in a particular lexical scope. That kind of
restricted-access package variable is going to be quite common in Perl
6 -- as an attribute of a class.

So the way we create such a variable is to declare it as a package
variable, but with the C<is private> property:

    module Wherever;

    sub whatever {
        our $count is private;
        $count++;
        print "whatever called $count times\n";
    }

Adding C<is private> causes Perl to recognize the existence of the
variable C<$count> within the C<Wherever> module, but then to restrict
its accessibility to the lexical scope in which it is first declared.
In the above example, any attempt to refer to C<$Wherever::count>
outside the C<&Wherever::whatever> subroutine produces a compile-time
error. It's still a package variable, but now you can't use it anywhere
but in the nominated lexical scope.

[Update: We now use C<state> variables for that.]

Apart from the benefit of replacing an ugly hack with a clean explicit
marker on the variable, the real advantage is that Perl 6 private
variables can be also be initialized:

    sub whatever {
        our $count is private //= 1;
        print "whatever called $count times\n";
        $count++;
    }

That initialization is performed the first time the variable
declaration is encountered during execution (because that's the only
time its value is C<undef>, so that's the only time the C<//=> operator
has any effect).

[Update: That's now just

    sub whatever {
        state $count = 1;
        say "whatever called $count times";
        $count++;
    }

The C<=> automatically happens only the first time through.]

In our example program we use that facility to do a one-time-only
initialization of a private package hash. That hash will then be used
as a (lexically restricted) look-up table to provide the
implementations for a set of operator symbols:

        our %operator is private //= (
            '*'  => { $^a * $^b },
            '/'  => { $^a / $^b },
            '~'  => { ($^a + $^b) / 2 },
        );

Each key of the hash is an operator symbol and the corresponding value
is an anonymous subroutine that implements the appropriate operation.
Note the use of the "place-holder" variables (C<$^a> and C<$^b>) to
implicitly specify the parameters of the closures.

Since all the data for the C<%operator> hash is constant, we could have
achieved a similar effect with:

        my %operator is constant = (
            '*'  => { $^a * $^b },
            '/'  => { $^a / $^b },
            '~'  => { ($^a + $^b) / 2 },
        );

Notionally this is quite different from the C<is private> version, in
that -- theoretically -- the lexical constant would be reconstructed
and reinitialized on each invocation of the C<calc> subroutine.
Although, in practice, we would expect the compiler to notice the
constant initializer and optimize the initialization out to
compile-time.

If the initializer had been a run-time expression, then the 
C<is private> and C<is constant> versions would behave very
differently:

    our %operator is private //= todays_ops();   # Initialize once, the first
                                                 # time statement is reached.
                                                 # Thereafter may be changed
                                                 # at will within subroutine.

    my %operator is constant = todays_ops();     # Re-initialize every time
                                                 # statement is reached.
                                                 # Thereafter constant
                                                 # within subroutine

[Update: The behavior of C<=> now always DWYMs from the declarator,
whether it's C<constant> (compile time), C<state> (first time), C<has>
(object initialization time), or C<our> or C<my> (execution time).]

=head1 Let's Split!

We then have to split the input expression into (whitespace-delimited)
tokens, in order to parse and execute it. Since the calculator language
we're implementing is RPN, we need a stack to store data and interim
calculations:

    my @stack;

We also need a counter to track the current token number (for error
messages):

    my $toknum = 1;

Then we just use the standard C<split> built-in to break up the
expression string, and iterate through each of the resulting tokens
using a C<for> loop:

    for split /\s+/, $expr -> $token {

There are several important features to note in this C<for> loop. To
begin with, there are no parentheses around the list. In Perl 6, they
are not required (they're not needed for I<any> control structure),
though they are certainly still permissible:

    for (split /\s+/, $expr) -> $token {

More importantly, the declaration of the iterator variable (C<$token>)
is no longer to the left of the list:

    # Perl 5 code
    for my $token (split /\s+/, $expr) {

Instead, it is specified via a topical arrow to the right of the list.

By the way, somewhat surprisingly, the Perl 6 arrow operator I<isn't> a
binary operator. (Actually, neither is the Perl 5 arrow operator, but
that's not important right now.)

Even more surprisingly, what the Perl 6 arrow operator is, is a synonym
for the declarator C<sub>. That's right, in Perl 6 you can declare an
anonymous subroutine like so:

    $product_plus_one = -> $x, $y { $x*$y + 1 };

The arrow behaves like an anonymous C<sub> declarator:

    $product_plus_one = sub($x, $y) { $x*$y + 1 };

except that its parameter list doesn't require parentheses. That
implies:

=over

=item *

The Perl 6 C<for>, C<while>, C<if>, and C<given> statements each take
two arguments: an expression that controls them and a
subroutine/closure that they execute. Normally, that closure is just a
block (in Perl6 I<all> blocks are really closures):

    for 1..10 {         # no comma needed before opening brace
        print
    }

but you can also be explicit:

    for 1..10, sub {    # needs comma if a regular anonymous sub
        print
    }

or you can be pointed:

    for 1..10 -> {      # no comma needed with arrow notation
        print
    }

or referential:

    for 1..10,          # needs comma if a regular sub reference
        &some_sub;

=item *

The variable after the arrow is effectively a lexical variable
confined to the scope of the following block (just as a subroutine
parameter is a lexical variable confined to the scope of the
subroutine block). Within the block, that lexical becomes an alias for
the topic (just as a subroutine parameter becomes an alias for the
corresponding argument).

=item *

Topic variables created with the arrow notation are, by default,
read-only aliases (because Perl 6 subroutine parameters are, by
default, read-only aliases):

    for @list -> $i {
        if ($cmd =~ 'incr') {
            $i++;   # Error: $i is read-only
        }
    }

Note that the rule doesn't apply to the default topic (C<$_>), which is
given special dispensation to be a modifiable alias (as in Perl 5).

=item *

If you want a named topic to be modifiable through its alias, then you
have to say so explicitly:

    for @list -> $i is rw {
        if ($cmd =~ 'incr') {
            $i++;   # Okay: $i is read-write
        }
    }

=item *

Just as a subroutine can have more than one parameter, so too we can
specify more than one named iterator variable at a time:

    for %phonebook.kv -> $name, $number {
        print "$name: $number\n"
    }

Note that in Perl 6, a hash in a list context returns a list of pairs,
not the Perl 5-ish "key, value, key, value, ..." sequence. To get the
hash contents in that format, we have to call the hash's C<kv> method
explicitly.

What actually happens in this iteration (and, in fact, in all such
instances) is that the C<for> loop looks at the number of arguments its
closure takes and iterates that many elements at a time.

Note that C<map> and C<reduce> can do that too in Perl 6:

    # process @xs_and_ys two-at-a-time...
    @list_of_powers = map { $^x ** $^y } @xs_and_ys;

[Update: A block parameter must now be followed by comma if there are 
more arguments.]

    # reduce list three-at-a-time   
    $sum_of_powers  = reduce { $^partial_sum + $^x ** $^y } 0, @xs_and_ys;

And, of course, since C<map> and C<reduce> take a subroutine reference
as their first argument -- instead of using the higher-order
placeholder notation -- we could use the arrow notation here too:

    @list_of_powers = map -> $x, $y { $x ** $y } @xs_and_ys;

or even an old-fashioned anonymous subroutine:

    @list_of_powers = map sub($x,$y){ $x ** $y }, @xs_and_ys;

=back

Phew. If that all makes your head hurt, then don't worry. All you
really need to remember is this: If you don't want to use C<$_> as the
name of the current topic, then you can change it by putting an arrow
and a variable name before the block of most control statements.

=head1 A Trying Situation

Once the calculator's input has been split into tokens, the C<for> loop
processes each one in turn, by applying them (if they represent an
operator), or jumping out of the loop (if they represent an
end-of-expression marker: C<'.'>, C<';'>, or C<'='>), or pushing them
onto the stack (since anything else must be an operand):

    try {
        when %operator {                # apply operator
            my @args = splice @stack, -2;
            push @stack, %operator{$token}(*@args);
        }

        when '.', ';', '=' {           # or jump out of loop
            last;
        }

        use fatal;
        push @stack, get_data($token);  # or push operand

The first two possibilities are tested for using C<when> statements.
Recall that a C<when> tests its first argument against the current
topic. In this case, however, the token was made the topic by the
surrounding C<for>. This is a significant feature of Perl 6: C<when>
blocks can implement a switch statement I<anywhere> there is a valid
topic, not just inside a C<given>.

The block associated with C<when %operator> will be selected if
C<%operator{$token}> is true (i.e. if there is an operator
implementation in C<%operator> corresponding to the current topic). In
that case, the top two arguments are spliced from the stack and passed
to the closure implementing that operation
(C<%operator{$token}(*@args)>). Note that there would normally be a dot
(C<.>) operator between the hash entry (i.e. a subroutine reference)
and the subroutine call, like so:

    %operator{$token}.(*@args)

but in Perl 6 it may be omitted since it can be inferred (just as an
inferrable C<< -> >> can be omitted in Perl 5).

Note too that we used the flattening operator (C<*>) on C<@args>,
because the closure returned by C<%operator{$token}> expects two
scalar arguments, not one array.

[Update: That would be the C<[,]> operator now.]

The second C<when> simply exits the loop if it finds an
"end-of-expression" token. In this example, the argument of the C<when>
is a list of strings, so the C<when> succeeds if any of them matches
the token.

Of course, since the entire body of the C<when> block is a single
statement, we could also have written the C<when> as a statement
modifier:

        last when '.', ';', '=';

The fact that C<when> has a postfix version like this should come as no
surprise, since C<when> is simply another control structure like C<if>,
C<for>, C<while>, etc.

The postfix version of C<when> does have one interesting feature. Since
it governs a statement, rather than a block, it does not provide the
block-C<when>'s automatic "C<break> to the end of my topicalizing
block" behavior. In this instance, it makes no difference since the
C<last> would do that anyway.

The final alternative -- pushing the token onto the stack -- is simply
a regular Perl C<push> command. The only interesting feature is that it
calls the C<get_data> subroutine to pre-translate the token if
necessary. It also specifies a C<use fatal> so that C<get_data> will
fail by an throwing exception, rather than returning C<undef>.

The loop tries each of these possibilities in turn. And "tries" is the
operative word here, because either the application of operations or
the pushing of data onto the stack may fail, resulting in an exception.
To prevent that exception from propagating all the way back to the main
program and terminating it, the various alternatives are placed in a
C<try> block.

A C<try> block is the Perl 6 successor to Perl 5's C<eval> block.
Unless it includes some explicit error handling code (see 
L<"Where's the Catch???">), it acts exactly like a Perl 5 
C<eval {...}>, intercepting a propagating exception and converting it
to an C<undef> return value:

    try { $quotient = $numerator / $denominator } // warn "couldn't divide";

=head1 Where's the Catch???

In Perl 6, we aren't limited to just blindly catching a propagating
exception and then coping with an C<undef>. It is also possible to set
up an explicit handler to catch, identify and deal with various types
of exceptions. That's done in a C<CATCH> block:

    CATCH {
        when Err::Reportable     { warn $!; continue }
        when Err::BadData        { $!.fail(at=>$toknum) }
        when NoData              { push @stack, 0 }
        when /division by zero/  { push @stack, Inf }
    }

A C<CATCH> block is like a C<BEGIN> block (hence the capitalization).
Its one argument is a closure that is executed if an exception ever
propagates as far as the block in which the C<CATCH> was declared. If
the block eventually executes, then the current topic is aliased to the
error variable C<$!>. So the typical thing to do is to populate the
exception handler's closure with a series of C<when> statements that
identify the exception contained in C<$!> and handle the error
appropriately. More on that L<in a moment|"Catch as Catch Can">.

The C<CATCH> block has one additional property. When its closure has
executed, it transfers control to the end of the block in which it was
defined. This means that exception handling in Perl 6 is
non-resumptive: once an exception is handled, control passes outward,
and the code that threw the exception is not automatically re-executed.

If we did want "try, try, try again" exception handling instead, then
we'd need to explicitly code a loop around the code we're trying:

    # generate exceptions (sometimes)
    sub getnum_or_die {
        given <> {                      # readline and make it the topic
            die "$_ is not a number"
                unless defined && /^\d+$/;
            return $_;
        }
    }
    # non-resumptive exception handling
    sub readnum_or_cry {
        return getnum_or_die;       # maybe generate an exception
        CATCH { warn $! }           # if so, warn and fall out of sub
    }
    # pseudo-resumptive
    sub readnum_or_retry {
        loop {                      # loop endlessly...
            return getnum_or_die;   #   maybe generate an exception
            CATCH { warn $! }       #   if so, warn and fall out of loop
        }                           #   (i.e. loop back and try again)
    }

Note that this isn't true resumptive exception handling. Control still
passes outward -- to the end of the C<loop> block. But then the C<loop>
reiterates, sending control back into C<getnum_or_die> for another
attempt.

[Update: Resumptive exception handling can be done in Perl 6, but only
with the cooperation of the code throwing the error.  If the exception
object contains a resumption continuation, that continuation may be
called to resume after the call to the throw.  In fact, some warnings are
simply exceptions that are printed and resumed by default.]

=head1 Catch as Catch Can

Within the C<CATCH> block, the example uses the standard Perl 6
exception handling technique: a series of C<when> statements. Those
C<when> statements compare their arguments against the current topic.
In a C<CATCH> block, that topic is always aliased to the error variable
C<$!>, which contains a reference to the propagating exception object.

The first three C<when> statements use a classname as their argument.
When matching a classname against an object, the C<=~> operator (and
therefore any C<when> statement) will call the object's C<isa> method,
passing it the classname. So the first three cases of the handler:

    when Err::Reportable   { warn $!; continue }
    when Err::BadData      { $!.fail(at=>$toknum) }
    when NoData            { push @stack, 0 }

are (almost) equivalent to:

    if $!.isa(Err::Reportable)  { warn $! }
    elsif $!.isa(Err::BadData)  { $!.fail(at=>$toknum) }
    elsif $!.isa(NoData)        { push @stack, 0 }

except far more readable.

[Update: Actually, smartmatch calls C<.does> rather than C<.isa> now since
that is defined to work for any type, not just classes.]

The first C<when> statement simply passes the exception object to
C<warn>. Since C<warn> takes a string as its argument, the exception
object's stringification operator (inherited from the standard
C<Exception> class) is invoked and returns an appropriate diagnostic
string, which is printed. The C<when> block then executes a C<continue>
statement, which circumvents the default "C<break> out of the
surrounding topicalizer block" semantics of the C<when>.

The second C<when> statement calls the propagating exception's C<fail>
method to cause C<calc> either to return or rethrow the exception,
depending on whether C<use fatal> was set. In addition, it passes some
extra information to the exception, namely the number of the token that
caused the problem.

The third C<when> statement handles the case where there is no cached
data corresponding to the calculator's C<"previous"> keyword, by simply
pushing a zero onto the stack.

The final case that the handler tests for:

    when /division by zero/  { push @stack, Inf }

uses a regex, rather than a classname. This causes the topic (i.e. the
exception) to be stringified and pattern-matched against the regex. As
mentioned above, by default, all exceptions stringify to their own
diagnostic string. So this part of the handler simply tests whether
that string includes the words "division by zero," in which case it
pushes the Perl 6 infinity value onto the stack.

=head1 One Dot Only

The C<CATCH> block handled bad data by calling the C<fail> method of
the current exception:

    when Err::BadData  { $!.fail(at=>$toknum) }

That's a particular instance of a far more general activity: calling a
method on the current topic. Perl 6 provides a shortcut for that -- the
prefix unary dot operator. Unary dot calls the method that is its
single operand, using the current topic as the implicit invocant. So
the C<Err::BadData> handler could have been written:

    when Err::BadData  { .fail(at=>$toknum) }

One of the main uses of unary dot is to allow C<when> statements to
select behavior on the basis of method calls. For example:

    given $some_object {
        when .has_data('new') { print "New data available\n" }
        when .has_data('old') { print "Old data still available\n" }
        when .is_updating     { sleep 1 }
        when .can('die')      { .die("bad state") }    # $some_object.die(...)
        default               { die "internal error" } # global die
    }

Unary dot is also useful within the definition of methods themselves.
In a Perl 6 method, the invocant (i.e. the first argument of the
method, which is a reference to the object on which the method was
invoked) is always the topic, so instead of writing:

    method dogtag (Soldier $self) {
        print $self.rank, " ", $self.name, "\n"
            unless $self.status('covert');
    }

we can just write:

    method dogtag (Soldier $self) {     # $self is automagically the topic
        print .rank, " ", .name, "\n"
            unless .status('covert');
    }

or even just:

    method dogtag {                     # @_[0] is automagically the topic
        print .rank, " ", .name, "\n"
            unless .status('covert');
    }

[Update: This is no longer the case unless you declare the invocant with
the name C<$_>.  Otherwise you have to say C<self.rank> or C<$.rank>.]

Yet another use of unary dot is as a way of abbreviating multiple
accesses to hash or array elements. That is, C<given> also implements
the oft-coveted C<with> statement. If many elements of a hash or array
are to be accessed in a set of statements, then we can avoid the
tedious repetition of the container name:

    # initialize from %options...

    $name  = %options{name} // %options{default_name};
    $age   = %options{age};
    $limit = max(%options{limit}, %options{rate} * %options{count});
    $count = $limit / %options{max_per_count};

by making it the topic and using unary dot:

    # initialize from %options...

    given %options {
        $name  = .{name} // .{default_name};
        $age   = .{age};
        $limit = max(.{limit}, .{rate} * .{count});
        $count = $limit / .{max_per_count};
    }

[Update: Would now be C<< .<name> >> etc., since C<.{...}> no longer
autoquotes.]

=head1 Onward and Backward

Back in our example, after each token has been dealt with in its loop
iteration, the iteration is finished. All that remains to do is
increment the token number.

In Perl 5, that would be done in a C<continue> block at the end of the
loop block. In Perl 6, it's done in a C<NEXT> statement I<within> the
loop block:

    NEXT { $toknum++ }

Like a C<CATCH>, a C<NEXT> is a special-purpose C<BEGIN> block that
takes a closure as its single argument. The C<NEXT> pushes that closure
onto the end of a queue of "next-iteration" handlers, all of which are
executed each time a loop reaches the end of an iteration. That is,
when the loop reaches the end of its block or when it executes an
explicit C<next> or C<last>.

The advantage of moving from Perl 5's external C<continue> to Perl 6's
internal C<NEXT> is that it gives the "next-iteration" handler access
to any lexical variables declared within the loop block. In addition,
it allows the "next-iteration" handler to be placed anywhere in the
loop that's convenient (e.g. close to the initialization it's later
supposed to clean up).

For example, instead of having to write:

    # Perl 5 code
    my $in_file, $out_file;
    while (<>) {
        open $in_file, $_ or die;
        open $out_file, "> $_.out" or die;

        # process files here (maybe next'ing out early)
    }
    continue {
        close $in_file  or die;
        close $out_file or die;
    }

we can just write:

    while (<>) {
        my $in_file  = open $_ or die;
        my $out_file = open "> $_.out" or die;
        NEXT {
            close $in_file  or die;
            close $out_file or die;
        }

        # process files here (maybe next'ing out early)
    }

There's no need to declare C<$in_file> and C<$out_file> outside the
loop, because they don't have to be accessible outside the loop (i.e.
in an external C<continue>).

This ability to declare, access and clean up lexicals within a given
scope is especially important because, in Perl 6, there is no reference
counting to ensure that the filehandles close themselves automatically
immediately at the end of the block. Perl 6's full incremental garbage
collector I<does> guarantee to eventually call the filehandle's
destructors, but makes no promises about when that will happen.

Note that there is also a C<LAST> statement, which sets up a handler
that is called automatically when a block is left for the last time.
For example, this:

    for reverse 1..10 {
        print "$_..." and flush;
        NEXT { sleep 1 }
        LAST { ignition() && print "lift-off!\n" }
    }

prints:

    10...9...8...7...6...5...4...3...2...1...lift-off!

sleeping one second after each iteration (including the last one), and
then calling C<&ignition> at the end of the countdown.

C<LAST> statements are also extremely useful in nonlooping blocks, as a
way of giving the block a "destructor" with which it can clean up its
state regardless of how it is exited:

[Update: This would now be a C<LEAVE> block.]

    sub handler ($value, $was_handled is rw) {
        given $value {
            LAST { $was_handled = 1 }
            when &odd { return "$value is odd" }
            when /0$/ { print "decimal compatible" }
            when /2$/ { print "binary compatible"; break }
            $value %= 7;
            when 1,3,5 { die "odd residual" }
        }
    }

In the above example, no matter how the C<given> block exits -- i.e.
via the C<return> of the first C<when> block, or via the (implicit)
C<break> of the second C<when>, or via the (explicit and redundant)
C<break> of the third C<when>, or via the C<"odd residual"> exception,
or by falling off the end of the C<given> block -- the C<$was_handled>
parameter is always correctly set.

Note that the C<LAST> is essential here. It wouldn't suffice to write:

    sub handler ($value, $was_handled is rw) {
        given $value {
            when &odd { return '$value is odd" }
            when /3$/ { print "ternary compatible" }
            when /2$/ { print "binary compatible"; break }
            $value %= 7;
            when 1,3,5 { die "odd residual" }
        }
        $was_handled = 1;
    }

because then C<$handled> wouldn't be set if an exception was thrown. Of
course, if that's actually the semantics you I<wanted>, then you don't
want C<LAST> in that case.

=head1 WHY ARE YOU SHOUTING???

You may be wondering why C<try> is in lower case but C<CATCH> is in
upper. Or why C<NEXT> and C<LAST> blocks have those "loud" keywords.

The reason is simple: C<CATCH>, C<NEXT> and C<LAST> blocks are just
specialized C<BEGIN> blocks that install particular types of handlers
into the block in which they appear.

They install those handlers at compile-time so, unlike a C<try> or a
C<next> or a C<last>, they don't actually I<do> anything when the
run-time flow of execution reaches them. The blocks associated with
them are only executed if the appropriate condition or exception is
encountered within their scope. And, if that happens, then they are
executed automatically, just like C<AUTOLOAD>, or C<DESTROY>, or
C<TIEHASH>, or C<FETCH>, etc.

So Perl 6 is merely continuing the long Perl tradition of using a
capitalized keyword to highlight code that is executed automatically.

In other words: I'M SHOUTING BECAUSE I WANT YOU TO BE AWARE THAT
SOMETHING SUBTLE IS HAPPENING AT THIS POINT.

=head1 Cache and Return

Meanwhile, back in C<calc>...

Once the loop is complete and all the tokens have been processed, the
result of the calculation should be the top item on the stack. If the
stack of items has more than one element left, then it's likely that
the expression was wrong somehow (most probably, because there were too
many original operands). So we report that:

    fail Err::BadData : msg=>"Too many operands"
        if @stack > 1;

If everything is OK, then we simply pop the one remaining value off the
stack and make sure it will evaluate true (even if its value is zero or
C<undef>) by setting its C<true> property. This avoids the potential
bug L<discussed earlier|"Still Other Whens">.

Finally, we record it in C<%var> under the key C<'$I<n>'> (i.e. as the
I<n>-th result), and return it:

    return %var{'$' _ $i} = pop(@stack) but true;

"But, but, but...", I hear you expostulate, "...shouldn't that be
C<pop(@stack) B<is> true>???"

Once upon a time, yes. But Larry has recently decided that compile-time
and run-time properties should have different keywords. Compile-time
properties (i.e. those ascribed to declarations) will still be
specified with the C<is> keyword:

    class Child is interface;
    my $heart is constant = "true";
    our $meeting is private;

whereas run-time properties (i.e. those ascribed to values) will now be
specified with the C<but> keyword:

    $str = <$trusted_fh> but tainted(0);
    $fh = open($filename) but chomped;
    return 0 but true;

The choice of C<but> is meant to convey the fact that run-time
properties will generally contradict some standard property of a value,
such as its normal truth, chompedness or tainting.

It's also meant to keep people from writing the very natural, but very
misguided:

    if ($x is true) {...}

which now generates a (compile-time) error:

    Can't ascribe a compile-time property to the run-time value of $x.
    (Did you mean "$x but true" or "$x =~ true"?)

[Update: The true value is now C<True>, short for C<Bool::True>.
This avoids confusion with the unary true() operator.]

=head1 The Forever Loop

Once the C<Calc> module has all its functionality defined, all that's
required is to write the main input-process-output loop. We'll cheat a
little and write it as an infinite loop, and then (in solemn Unix
tradition) we'll require an EOF signal to exit.

The infinite loop needs to keep track of its iteration count. In Perl 5
that would be:

    # Perl 5 code
    for (my $i=0; 1; $i++) {

which would translate into Perl 6 as:

    loop (my $i=0; 1; $i++) {

since Perl 5's C-like C<for> loop has been renamed C<loop> in Perl 6 --
to distinguish it from the Perl-like C<for> loop.

However, Perl 6 also allows us to create semi-infinite, lazily
evaluated lists, so we can write the same loop much more cleanly as:

    for 0..Inf -> $i {

When C<Inf> is used as the right-hand operand to C<..>, it signifies
that the resulting list must be lazily built, and endlessly iterable.
This type of loop will probably be common in Perl 6 as an easy way of
providing a loop counter.

If we need to iterate some list of values, as well as tracking a loop
counter, then we can take advantage of another new feature of Perl 6:
iteration streams.

A regular Perl 6 C<for> loop iterates a single stream of values,
aliasing the current topic to each in turn:

    for @stream -> $topic_from_stream {
        ...
    }

But it's also possible to specify two (or more) streams of values that
the one C<for> loop will step through I<in parallel>:

    for @stream1 ; @stream2 -> $topic_from_stream1 ; $topic_from_stream2 {
        ...
    }

Each stream of values is separated by a semicolon, and each topic
variable is similarly separated. The C<for> loop iterates both streams
in parallel, aliasing the next element of the first stream
(C<@stream1>) to the first topic (C<$topic_from_stream1>) and the next
element of the second stream (C<@stream2>) to the second topic
(C<$topic_from_stream2>).

The commonest application of this will probably be to iterate a list
and simultaneously provide an iteration counter:

    for @list; 0..@list.last -> $next; $index {
        print "Element $index is $next\n";
    }

It may be useful to set that out slightly differently, to show the
parallel nature of the iteration:

    for  @list ; 0..@list.last
     ->  $next ; $index   {
        print "Element $index is $next\n";
    }

It's important to note that writing:

    for @a; @b -> $x; $y {...}
    # in parallel, iterate @a one-at-a-time as $x, and @b one-at-a-time as $y

is I<not> the same as writing:

    for @a, @b -> $x, $y {...}
    # sequentially iterate @a then @b, two-at-a-time as $x and $y

The difference is that semicolons separate streams, while commas
separate elements within a single stream.

If we were brave enough, then we could even combine the two:

    for @a1, @a2; @b -> $x; $y1, $y2 {...}
    # sequentially iterate @a1 then @a2, one-at-a-time as $x
    # and, in parallel, iterate @b two-at-a-time as $y1 and $y2

This is definitely a case where a different layout would help make the
various iterations and topic bindings clearer:

    for @a1, @a2 ;  @b
     -> $x       ;  $y1, $y2   {...}

Note, however, that the normal way in Perl 6 to step through an array's
values while tracking its indices will almost certainly be to use the
array's C<kv> method. That method returns a list of interleaved indices
and values (much like the hash's C<kv> method returns alternating keys
and values):

    for @list.kv -> $index, $next {
        print "Element $index is $next\n";
    }

[Update: Most of the syntax in this section is bogus.  There is
no semicolon separator in argument lists.  Instead the mapping is
controlled by the function around the argument list, such as each(),
zip(), and so on.  These functions might use semicolon to separate
separate dimensions, but would normally hide those inside parens to
prevent confusion with statement ending semicolons.]

=head1 Read or Die

Having prompted for the next expression that the calculator will
evaluate:

    print "$i> ";

we read in the expression and check for an EOF (which will cause the
C<< <> >> operator to return C<undef>, in which case we escape the
infinite loop):

    my $expr = <> err last;

Err...C<err>???

In Apocalypse 3, Larry introduced the C<//> operator, which is like a
C<||> that tests its left operand for definedness rather than truth.

What he didn't mention (but which you probably guessed) was that there
is also the low-precedence version of C<//>. Its name is C<err>:

          Operation         High Precedence       Low Precedence

         INCLUSIVE OR             ||                     or
         EXCLUSIVE OR             ~~                    xor
          DEFINED OR              //                    err

[Update: High precedence XOR is now ^^.]

But why call it C<err>?

Well, the C<//> operator looks like a skewed version of C<||>, so the
low-precedence version should probably be a skewed version of C<or>. We
can't skew it visually (even Larry thought that using italics would be
going a bit far), so we skew it phonetically instead: 
C<or> -> C<err>.

C<err> also has the two handy mnemonic connotations:

=over

=item *

That we're handling an B<err>or marker (which a returned C<undef>
usually is)

=item *

That we're voicing a surprised double-take after something
unexpected (which a returned C<undef> often is).

=back

Besides all that, it just seems to work well. That is, something like
this:

    my $value = compute_value(@args)
        err die "Was expecting a defined value";

reads quite naturally in English (whether you think of C<err> as an
abbreviation of "on error...", or as a synonym for "oops...").

Note that C<err> is a binary operator, just like C<or>, and C<xor>, so
there's no particular need to start it on a new line:

    my $value = compute_value(@args) err die "Was expecting a defined value";

In our example program, the C<undef> returned by the C<< <> >>
operator at end-of-file is our signal to jump out of the main loop. To
accomplish that we simply append C<err last> to the input statement:

    my $expr = <> err last;

Note that an C<or last> wouldn't work here, as both the empty string
and the string "0" are valid (i.e. non-terminating) inputs to the
calculator.

=head1 Just Do It

Then it's just a matter of calling C<Calc::calc>, passing it the
iteration number and the expression:

    Calc::calc(i=>$i, expr=>$expr)

Note that we used named arguments, so passing them in the wrong order
didn't matter.

We then interpolate the result back into the output string using the
C<$(...)> scalar interpolator:

    print "$i> $( Calc::calc(i=>$i, expr=>$expr) )\n";

We could even simplify that a little further, by taking advatage of the
fact that subroutine calls interpolate directly into strings in Perl 6,
provided we use the C<&> prefix:

    print "$i> &Calc::calc(i=>$i, expr=>$expr)\n";

Either way, that's it: we're done.

=head1 Summing Up

In terms of control structures, Perl 6:

=over

=item * 

provides far more support for exceptions and exception handling,

=item *

cleans up and extends the C<for> loop syntax in several ways,

=item *

unifies the notions of blocks and closures and makes them interchangeable,

=item *

provides hooks for attaching various kinds of automatic handlers to a
block/closure,

=item *

re-factors the concept of a switch statement into two far more general ideas:
marking a value/variable as the current topic, and then doing "smart matching"
against that topic.

=back

These extensions and cleanups offer us far more power and control, and
-- amazingly -- in most cases require far less syntax. For example,
here's (almost) the same program, written in Perl 5:

    package Err::BadData; 
    use base 'Exception';   # which you'd have to write yourself

    package NoData;         # not lexical
    use base 'Exception';
    sub warn { die @_ }

    package Calc;

    my %var;

    sub get_data  {
        my $data = shift;
        if ($data =~ /^\d+$/)       { return $var{""} = $data }
        elsif ($data eq 'previous') { return defined $var{""}
                                                 ? $var{""}
                                                 : die NoData->new() 
                                    }
        elsif ($var{$data})         { return $var{""} = $var{$data} }
        else                        { die Err::BadData->new(
                                             {msg=>"Don't understand $data"}
                                          )
                                     }
    }

    sub calc {
        my %data = @_;
        my ($i, $expr) = @data{'i', 'expr'};
        my %operator = (
            '*'  => sub { $_[0] * $_[1] },
            '/'  => sub { $_[0] / $_[1] },
            '~'  => sub { ($_[0] + $_[1]) / 2 },
        );

        my @stack;
        my $toknum = 1;
        LOOP: for my $token (split /\s+/, $expr) {
            defined eval {
                TRY: if ($operator{$token}) {
                    my @args = splice @stack, -2;
                    push @stack, $operator{$token}->(@args);
                    last TRY;
                }
                last LOOP if $token eq '.' || $token eq ';' || $token eq '=';

                push @stack, get_data($token);
            } || do {
                if ($@->isa(Err::Reportable))     { warn $@; }
                if ($@->isa(Err::BadData))        { $@->{at} = $i; die $@ }
                elsif ($@->isa(NoData))           { push @stack, 0     }
                elsif ($@ =~ /division by zero/)  { push @stack, ~0 }
            }
        }
        continue { $toknum++ }
        die Err::BadData->new(msg=>"Too many operands") if @stack > 1;
        $var{'$'.$i} = $stack[-1] . ' but true';
        return 0+pop(@stack);
    }

    package main;

    for (my $i=1; 1; $i++) {
        print "$i> ";
        defined( my $expr = <> ) or last;
        print "$i> ${\Calc::calc(i=>$i, expr=>$expr)}\n";
    }

Hmmmmmmm. I know which version I<I'd> rather maintain.
