control.pod6

=begin pod :tag<perl6>

=TITLE Control flow

=SUBTITLE Statements used to control the flow of execution

=head1 X<statements|control flow, statements>

Perl 6 programs consist of one or more statements.  Simple statements
are separated by semicolons.  The following program will say "Hello"
and then say "World" on the next line.

    say "Hello";
    say "World";

In most places where spaces appear in a statement, and before the
semicolon, it may be split up over many lines.  Also, multiple statements
may appear on the same line.  It would be awkward, but the above could
also be written as:

    say
    "Hello"; say "World";

X<blocks|control flow, blocks>
=head1 Blocks

Like many languages, Perl 6 uses C<blocks> enclosed by C<{> and C<}> to turn
multiple statements into a single statement.  It is OK to skip the semicolon
between the last statement in a block and the closing C<}>.

    { say "Hello"; say "World" }

When a block stands alone as a statement, it will be entered immediately
after the previous statement finishes, and the statements inside it will be
executed.

    say 1;                    # OUTPUT: «1␤»
    { say 2; say 3 };         # OUTPUT: «2␤3␤»
    say 4;                    # OUTPUT: «4␤»

Unless it stands alone as a statement, a block simply creates a closure.  The
statements inside are not executed immediately.  Closures are another topic
and how they are used is explained
L<elsewhere|/language/functions#Blocks_and_lambdas>.  For now it is just
important to understand when blocks run and when they do not:

=for code
say "We get here"; { say "then here." }; { say "not here"; 0; } or die;

In the above example, after running the first statement, the first block stands
alone as a second statement, so we run the statement inside it.  The second
block does not stand alone as a statement, so instead, it makes an object of
type C<Block> but does not run it.  Object instances are usually considered to
be true, so the code does not die, even though that block would evaluate to 0,
were it to be executed.  The example does not say what to do with the C<Block>
object, so it just gets thrown away.

Most of the flow control constructs covered below are just ways to tell perl 6
when, how, and how many times, to enter blocks like that second block.

Before we go into those, an important side-note on syntax: If there is
nothing (or nothing but comments) on a line after a closing curly brace where
you would normally put semicolon, then you do not need the semicolon:

    # All three of these lines can appear as a group, as is, in a program
    { 42.say }                # OUTPUT: «42␤»
    { 43.say }                # OUTPUT: «43␤»
    { 42.say }; { 43.say }    # OUTPUT: «42␤43␤»

...but:

=begin code :skip-test<syntax error>
{ 42.say }  { 43.say }    # Syntax error
{ 42.say; } { 43.say }    # Also a syntax error, of course
=end code

So, be careful when you backspace in a line-wrapping editor:

=begin code :skip-test<syntax error>
{ "Without semicolons line-wrapping can be a bit treacherous.".say } \
{ 43.say } # Syntax error
=end code

You have to watch out for this in most languages anyway to prevent things
from getting accidentally commented out.  Many of the examples below may
have unnecessary semicolons for clarity.

Class bodies behave like simple blocks for any top level expression; same goes
to roles and other packages, like grammars (which are actually classes)
or modules.

=begin code :skip-test<Fails>
class C {
    say "I live";
    die "I will never live!"
};
my $c = C.new;                              │
# OUTPUT: Fails and writes «I live␤I will never live!␤
=end code

This block will first run the first statement, and then C<die> printing the
second statement. C<$c> will never get a value.

=head1 X<Phasers|control flow,Phasers>

Blocks may have I<phasers>: special labeled blocks that break their execution
into phases that run in particular phases. See the page
L<phasers|/language/phasers> for the details.

=head1 X<do|control flow,do>

The simplest way to run a block where it cannot be a stand-alone statement
is by writing C<do> before it:

=for code
# This dies half of the time
do { say "Heads I win, tails I die."; Bool.pick } or die; say "I win.";

Note that you need a space between the C<do> and the block.

The whole C<do {...}> evaluates to the final value of the block.  The block
will be run when that value is needed in order to evaluate the rest of the
expression.  So:

    False and do { 42.say };

...will not say 42.  However, the block is only evaluated once each time
the expression it is contained in is evaluated:

    # This says "(..1 ..2 ..3)" not "(..1 ...2 ....3)"
    my $f = "."; say do { $f ~= "." } X~ 1, 2, 3;

In other words, it follows the same reification rules as everything else.

Technically, C<do> is a loop which runs exactly one iteration.

A C<do> may also be used on a bare statement (without curly braces)
but this is mainly just useful for avoiding the syntactical need to
parenthesize a statement if it is the last thing in an expression:

=for code
3, do if 1 { 2 }  ; # OUTPUT: «(3, 2)␤»
3,   (if 1 { 2 }) ; # OUTPUT: «(3, 2)␤»
=for code :skip-test<syntax error>
3,    if 1 { 2 }  ; # Syntax error

=head1 X<start|control flow,start>

The simplest way to run a block B<asynchronously> is by writing C<start>
before it:

=for code
start { sleep 1; say "done" }
say "working";
# working, done

Note that you need a space between the C<start> and the block. In the example above, the C<start> block is in sink context, since it's not assigned to a variable. From version 6.d, these kind of blocks have an exception handler attached:

=for code :skip-test<Illustrates exception>
start { die "We're dead"; }
say "working";
sleep 10;

This code will print C<Unhandled exception in code scheduled on thread 4 We're
dead> in version 6.d, while it will simply get out after waiting for 10 seconds
in version 6.c. 


The C<start {...}> immediately returns a C<Promise> that can be safely ignored
if you are not interested in the result of the block.  If you B<are> interested
in the final value of the block, you can call the C<.result> method on the
returned promise.  So:

    my $promise = start { sleep 10; 42 }
    # ... do other stuff
    say "The result is $promise.result()";

If the code inside the block has not finished, the call to C<.result> will
wait until it is done.

A C<start> may also be used on a bare statement (without curly braces).
This is mainly just useful when calling a subroutine / method on an object
is the only thing to do asynchronously.

=head1 X<if|control flow,if>

To conditionally run a block of code, use an C<if> followed by a condition.
The condition, an expression, will be evaluated immediately after the
statement before the C<if> finishes.  The block attached to the condition will
only be evaluated if the condition means True when coerced to C<Bool>.
Unlike some languages the condition does not have to be parenthesized,
instead the C<{> and C<}> around the block are mandatory:

=for code
if 1 { "1 is true".say }  ; # says "1 is true"
=for code :skip-test<syntax error>
if 1   "1 is true".say    ; # syntax error, missing block
=for code
if 0 { "0 is true".say }  ; # does not say anything, because 0 is false
=for code
if 42.say and 0 { 43.say }; # says "42" but does not say "43"

There is also a form of C<if> called a "statement modifier" form.  In this
case, the if and then the condition come after the code you want to run
conditionally.  Do note that the condition is still always evaluated first:

    43.say if 42.say and 0;     # says "42" but does not say "43"
    43.say if 42.say and 1;     # says "42" and then says "43"
    say "It is easier to read code when 'if's are kept on left of screen"
        if True;                # says the above, because it is true
    { 43.say } if True;         # says "43" as well

The statement modifier form is probably best used sparingly.

The C<if> statement itself will either L<slip|/type/Slip> us an empty list, if
it does not run the block, or it will return the value which the block produces:

    my $d = 0; say (1, (if 0 { $d += 42; 2; }), 3, $d); # says "(1 3 0)"
    my $c = 0; say (1, (if 1 { $c += 42; 2; }), 3, $c); # says "(1 2 3 42)"
    say (1, (if 1 { 2, 2 }), 3);         # does not slip, says "(1 (2 2) 3)"

For the statement modifier it is the same, except you have the value
of the statement instead of a block:

    say (1, (42 if True) , 2); # says "(1 42 2)"
    say (1, (42 if False), 2); # says "(1 2)"
    say (1,  42 if False , 2); # says "(1 42)" because "if False, 2" is true

The C<if> does not change the topic (C<$_>) by default.  In order to access
the value which the conditional expression produced, you have to ask
for it more strongly:

    $_ = 1; if 42 { $_.say }                ; # says "1"
    $_ = 1; if 42 -> $_ { $_.say }          ; # says "42"
    $_ = 1; if 42 -> $a { $_.say;  $a.say } ; # says "1" then says "42"
    $_ = 1; if 42       { $_.say; $^a.say } ; # says "1" then says "42"

=head2 X<else/elsif|control flow,else elsif>

A compound conditional may be produced by following an C<if> conditional
with C<else> to provide an alternative block to run when the conditional
expression is false:

=for code
if 0 { say "no" } else { say "yes" }   ; # says "yes"
if 0 { say "no" } else{ say "yes" }    ; # says "yes", space is not required

The C<else> cannot be separated from the conditional statement by a
semicolon, but as a special case, it is OK to have a newline.

=for code :skip-test<syntax error>
if 0 { say "no" }; else { say "yes" }  ; # syntax error
=for code
if 0 { say "no" }
else { say "yes" }                     ; # says "yes"

Additional conditions may be sandwiched between the C<if> and the C<else> using
C<elsif>.  An extra condition will only be evaluated if all the conditions
before it were false, and only the block next to the first true condition will
be run.  You can end with an C<elsif> instead of an C<else> if you want.

    if 0 { say "no" } elsif False { say "NO" } else { say "yes" } # says "yes"
    if 0 { say "no" } elsif True { say "YES" } else { say "yes" } # says "YES"

    if 0 { say "no" } elsif False { say "NO" } # does not say anything

    sub right { "Right!".say; True }
    sub wrong { "Wrong!".say; False }
    if wrong() { say "no" } elsif right() { say "yes" } else { say "maybe" }
    # The above says "Wrong!" then says "Right!" then says "yes"

You cannot use the statement modifier form with C<else> or C<elsif>:

=for code :skip-test<syntax error>
42.say if 0 else { 43.say }            # syntax error

All the same rules for semicolons and newlines apply, consistently

=for code :skip-test<syntax error>
if 0 { say 0 }; elsif 1 { say 1 }  else { say "how?" } ; # syntax error
if 0 { say 0 }  elsif 1 { say 1 }; else { say "how?" } ; # syntax error

=for code
if 0 { say 0 }  elsif 1 { say 1 }  else { say "how?" } ; # says "1"

    if 0 { say 0 } elsif 1 { say 1 }
    else { say "how?" }                                    ; # says "1"

    if 0 { say 0 }
    elsif 1 { say 1 } else { say "how?" }                  ; # says "1"

    if        0 { say "no" }
    elsif False { say "NO" }
    else        { say "yes" }                              ; # says "yes"

The whole thing either L<slips|/type/Slip> us an empty list (if no blocks
were run) or returns the value produced by the block that did run:

    my $d = 0; say (1,
                    (if 0 { $d += 42; "two"; } elsif False { $d += 43; 2; }),
                    3, $d); # says "(1 3 0)"
    my $c = 0; say (1,
                    (if 0 { $c += 42; "two"; } else { $c += 43; 2; }),
                    3, $c); # says "(1 2 3 43)"

It's possible to obtain the value of the previous expression inside an
C<else>, which could be from C<if> or the last C<elsif> if any are
present:

    $_ = 1; if 0     { } else -> $a { "$_ $a".say } ; # says "1 0"
    $_ = 1; if False { } else -> $a { "$_ $a".say } ; # says "1 False"

    if False { } elsif 0 { } else -> $a { $a.say }  ; # says "0"

=head2 X<unless|control flow, unless>

When you get sick of typing "if not (X)" you may use C<unless> to invert
the sense of a conditional statement.  You cannot use C<else> or C<elsif>
with C<unless> because that ends up getting confusing.  Other than those
two differences C<unless> works the same as L<#if>:

=for code
unless 1 { "1 is false".say }  ; # does not say anything, since 1 is true
=for code :skip-test<syntax error>
unless 1   "1 is false".say    ; # syntax error, missing block
=for code
unless 0 { "0 is false".say }  ; # says "0 is false"

    unless 42.say and 1 { 43.say } ; # says "42" but does not say "43"
    43.say unless 42.say and 0;      # says "42" and then says "43"
    43.say unless 42.say and 1;      # says "42" but does not say "43"

    $_ = 1; unless 0 { $_.say }           ; # says "1"
    $_ = 1; unless 0 -> $_ { $_.say }     ; # says "0"
    $_ = 1; unless False -> $a { $a.say } ; # says "False"

    my $c = 0; say (1, (unless 0 { $c += 42; 2; }), 3, $c); # says "(1 2 3 42)"
    my $d = 0; say (1, (unless 1 { $d += 42; 2; }), 3, $d); # says "(1 3 0)"

X<with|control flow,with>
X<orwith|control flow,orwith>
X<without|control flow,without>
=head2 C<with>, C<orwith>, C<without>

The C<with> statement is like C<if> but tests for definedness rather than
truth.  In addition, it topicalizes on the condition, much like C<given>:

    with "abc".index("a") { .say }      # prints 0

Instead of C<elsif>, C<orwith> may be used to chain definedness tests:

    # The below code says "Found a at 0"
    my $s = "abc";
    with   $s.index("a") { say "Found a at $_" }
    orwith $s.index("b") { say "Found b at $_" }
    orwith $s.index("c") { say "Found c at $_" }
    else                 { say "Didn't find a, b or c" }

You may intermix C<if>-based and C<with>-based clauses.

    # This says "Yes"
    if 0 { say "No" } orwith Nil { say "No" } orwith 0 { say "Yes" };

As with C<unless>, you may use C<without> to check for undefinedness,
but you may not add an C<else> clause:

    my $answer = Any;
    without $answer { warn "Got: {$_.perl}" }

There are also C<with> and C<without> statement modifiers:

    my $answer = (Any, True).roll;
    say 42 with $answer;
    warn "undefined answer" without $answer;

=head1 X<when|control flow, when>

The C<when> block is similar to an C<if> block and either or both can be used in
an outer block; they also both have a "statement modifier" form. But there is a
difference in how following code in the same, outer block is handled: When the
C<when> block is executed, control is passed to the enclosing block and
following statements are ignored; but when the C<if> block is executed,
following statements are executed. N<There are other ways to modify their
default behavior; they are discussed in other sections.> The following
examples should illustrate the C<if> or C<when> block's default behavior
assuming no special exit or other side effect statements are included in the
C<if> or C<when> blocks:

=begin code
{
    if X {...} # if X is true in boolean context, block is executed
    # following statements are executed regardless
}
{
    when X {...} # if X is true in boolean context, block is executed
                 # and control passes to the outer block
    # following statements are NOT executed
}
=end code

Should the C<if> and C<when> blocks above appear at file scope, following
statements would be executed in each case.

There is one other feature C<when> has that C<if> doesn't: the C<when>'s
boolean context test defaults to C<$_ ~~> while the C<if>'s does not.  That has
an effect on how one uses the X in the C<when> block without a value for C<$_>
(it's C<Any> in that case and C<Any> smartmatches on C<True>: C<Any ~~ True>
yields C<True>).  Consider the following:

=begin code
{
    my $a = 1;
    my $b = True;
    when $a    { say 'a' }; # no output
    when so $a { say 'a' }  # a (in "so $a" 'so' coerces $a to Boolean context True
                            # which matches with Any)
    when $b    { say 'b' }; # no output (this statement won't be run)
}
=end code

Finally, C<when>'s statement modifier form does not effect execution
of following statements either inside or outside of another block:

=begin code
say "foo" when X; # if X is true statement is executed
                  # following statements are not affected
=end code

Since a successful match will exit the block, the behavior of this piece of
code:
=begin code
$_ = True;
my $a;
{
    $a = do when .so { "foo" }
};
say $a; # OUTPUT: «(Any)␤»
=end code

is explained since the C<do> block is abandoned before any value is stored or
processed. However, in this case:
=begin code
$_ = False;
my $a;
{
    $a = do when .so { "foo" }
};
say $a; # OUTPUT: «False␤»
=end code
the block is not abandoned since the comparison is false, so C<$a> will actually
get a value.

=head1 X<for|control flow, for>

The C<for> loop iterates over a list, running the statements inside a
L<block|/type/Block>
once on each iteration. If the block takes parameters, the elements of the
list are provided as arguments.

    my @foo = 1..3;
    for @foo { $_.print } # prints each value contained in @foo
    for @foo { .print }   # same thing, because .print implies a $_ argument
    for @foo { 42.print } # prints 42 as many times as @foo has elements

Pointy block syntax or a L<placeholder|/language/variables#The_^_twigil>
may be used to name the parameter, of course.

    my @foo = 1..3;
    for @foo -> $item { print $item }
    for @foo { print $^item }            # same thing

Multiple parameters can be declared, in which case the iterator takes
as many elements from the list as needed before running the block.

    my @foo = 1..3;
    for @foo.kv -> $idx, $val { say "$idx: $val" }
    my %hash = <a b c> Z=> 1,2,3;
    for %hash.kv -> $key, $val { say "$key => $val" }
    for 1, 1.1, 2, 2.1 { say "$^x < $^y" }  # says "1 < 1.1" then says "2 < 2.1"

Parameters of a pointy block can have default values, allowing to handle lists
with missing elements.

    my @list = 1,2,3,4;
    for @list -> $a, $b = 'N/A', $c = 'N/A' {
        say "$a $b $c"
    }
    # OUTPUT: «1 2 3␤4 N/A N/A␤»

If the postfix form of C<for> is used a block is not required and the topic is
set for the statement list.

    say „I $_ butterflies!“ for <♥ ♥ ♥>;
    # OUTPUT«I ♥ butterflies!␤I ♥ butterflies!␤I ♥ butterflies!␤»

A C<for> may be used on lazy lists – it will only take elements from the
list when they are needed, so to read a file line by line, you could
use:

=for code
for $*IN.lines -> $line { .say }

Iteration variables are always lexical, so you don't need to use C<my> to give
them the appropriate scope. Also, they are read-only aliases. If you need them
to be read-write, use C«<->» instead of C«->». If you need to make C<$_>
read-write in a for loop, do so explicitly.

    my @foo = 1..3;
    for @foo <-> $_ { $_++ }

A for loop can produce a C<List> of the values produced by each run of the
attached block.  To capture these values, put the for loop in parenthesis or
assign them to an array:

    (for 1, 2, 3 { $_ * 2 }).say;              # OUTPUT «(2 4 6)␤»
    my @a = do for 1, 2, 3 { $_ * 2 }; @a.say; # OUTPUT «[2 4 6]␤»
    my @b = (for 1, 2, 3 { $_ * 2 }); @b.say;  # OUTPUT: «[2 4 6]␤»

=head1 X<gather/take|control flow,gather take>

X<|lazy list,gather>X<|lazy list,take>

C<gather> is a statement or block prefix that returns a L<sequence|/type/Seq>
of values. The values come from calls to L<take|/type/Mu#routine_take> in the
dynamic scope of the C<gather> block.

    my @a = gather {
        take 1;
        take 5;
        take 42;
    }
    say join ', ', @a;          # OUTPUT: «1, 5, 42␤»

C<gather/take> can generate values lazily, depending on context. If you want to
force lazy evaluation use the L<lazy|/type/Iterable#method_lazy> subroutine or
method. Binding to a scalar or sigilless container will also force laziness.

For example

    my @vals = lazy gather {
        take 1;
        say "Produced a value";
        take 2;
    }
    say @vals[0];
    say 'between consumption of two values';
    say @vals[1];

    # OUTPUT:
    # 1
    # between consumption of two values
    # Produced a value
    # 2


C<gather/take> is scoped dynamically, so you can call C<take> from subs or
methods that are called from within C<gather>:

    sub weird(@elems, :$direction = 'forward') {
        my %direction = (
            forward  => sub { take $_ for @elems },
            backward => sub { take $_ for @elems.reverse },
            random   => sub { take $_ for @elems.pick(*) },
        );
        return gather %direction{$direction}();
    }

    say weird(<a b c>, :direction<backward> );          # OUTPUT: «(c b a)␤»

If values need to be mutable on the caller side, use L<take-rw|/type/Mu#routine_take-rw>.

Note that C<gather/take> also work for hashes. The return value is
still a C<Seq> but the assignment to a hash in the following example
makes it a hash.

    my %h = gather { take "foo" => 1; take "bar" => 2};
    say %h;                                             # OUTPUT: «{bar => 2, foo => 1}␤»

=head1 X<supply/emit|control flow,supply emit>

Emits the invocant into the enclosing
L<supply|/language/concurrency#index-entry-supply_(on-demand)>:

    my $supply = supply {
        emit $_ for "foo", 42, .5;
    }
    $supply.tap: {
        say "received {.^name} ($_)";
    }

    # OUTPUT:
    # received Str (foo)
    # received Int (42)
    # received Rat (0.5)

=head1 X<given|control flow, given>
X<|switch (given)>

The C<given> statement is Perl 6's topicalizing keyword in a similar way that
C<switch> topicalizes in languages such as C.  In other words, C<given>
sets C<$_> inside the following block.  The keywords for individual cases
are C<when> and C<default>.  The usual idiom looks like this:

    my $var = (Any, 21, any <answer lie>).pick;
    given $var {
        when 21 { say $_ * 2 }
        when 'lie' { .say }
        default { say 'default' }
    }

The C<given> statement is often used alone:

    given 42 { .say; .Numeric; }

This is a lot more understandable than:

    { .say; .Numeric; }(42)


=head2 X<default and when|control flow,default when>

A block containing a C<default> statement will be left immediately
when the sub-block after the C<default> statement is left.  It is
as though the rest of the statements in the block are skipped.

    given 42 {
        "This says".say;
        $_ == 42 and ( default { "This says, too".say; 43; } );
        "This never says".say;
    }
    # The above block evaluates to 43

A C<when> statement will also do this (but a C<when> statement modifier
will I<not>.)

In addition, C<when> statements C<smartmatch> the topic (C<$_>) against
a supplied expression such that it is possible to check against values,
regular expressions, and types when specifying a match.

    for 42, 43, "foo", 44, "bar" {
        when Int { .say }
        when /:i ^Bar/ { .say }
        default  { say "Not an Int or a Bar" }
    }
    # OUTPUT: «42␤43␤Not an Int or a Bar␤44␤Bar␤»

In this form, the C<given>/C<when> construct acts much like a set of
C<if>/C<elsif>/C<else> statements.  Be careful with the order of the
C<when> statements.  The following code says C<"Int"> not C<42>.

    given 42 {
        when Int { say "Int" }
        when 42  { say 42 }
        default  { say "huh?" }
    }
    # OUTPUT: «Int␤»

When a C<when> statement or C<default> statement causes the outer
block to return, nesting C<when> or C<default> blocks do not count
as the outer block, so you can nest these statements and still
be in the same "switch" just so long as you do not open a new block:

    given 42 {
        when Int {
          when 42  { say 42 }
          say "Int"
        }
        default  { say "huh?" }
    }
    # OUTPUT: «42»

C<when> statements can smartmatch against L<Signatures|/language/syntax#Signature_literals>.

=head2 X<proceed|control flow, proceed>
=head2 X<succeed|control flow, succeed>

Both C<proceed> and C<succeed> are meant to be used only from inside C<when>
or C<default> blocks.

The C<proceed> statement will immediately leave the C<when> or C<default>
block, skipping the rest of the statements, and resuming after the block.
This prevents the C<when> or C<default> from exiting the outer block.

=for code
given * {
    default {
        proceed;
        "This never says".say
    }
}
"This says".say;

This is most often used to enter multiple C<when> blocks.  C<proceed> will
resume matching after a successful match, like so:

    given 42 {
        when Int   { say "Int"; proceed }
        when 42    { say 42 }
        when 40..* { say "greater than 40" }
        default    { say "huh?" }
    }
    # OUTPUT: «Int␤»
    # OUTPUT: «42␤»

Note that the C<when 40..*> match didn't occur.  For this to match
such cases as well, one would need a C<proceed> in the C<when 42> block.

This is not like a C<C> C<switch> statement, because the C<proceed> does
not merely enter the directly following block, it attempts to match
the C<given> value once more, consider this code:

    given 42 {
        when Int { "Int".say; proceed }
        when 43  { 43.say }
        when 42  { 42.say }
        default  { "got change for an existential answer?".say }
    }
    # OUTPUT: «Int␤»
    # OUTPUT: «42␤»

...which matches the C<Int>, skips C<43> since the value doesn't match, matches
C<42> since this is the next positive match, but doesn't enter the
C<default> block since the C<when 42> block doesn't contain a C<proceed>.

By contrast, the C<succeed> keyword short-circuits execution and exits the
entire C<given> block at that point.  It may also take an argument to
specify a final value for the block.

    given 42 {
        when Int {
            say "Int";
            succeed "Found";
            say "never this!";
        }
        when 42 { say 42 }
        default { say "dunno?" }
    }
    # OUTPUT: «Int␤»

If you are not inside a when or default block, it is an error to try
to use C<proceed> or C<succeed>.  Also remember, the C<when> statement
modifier form does not cause any blocks to be left, and any C<succeed>
or C<proceed> in such a statement applies to the surrounding clause,
if there is one:

    given 42 {
        { say "This says" } when Int;
        "This says too".say;
        when * > 41 {
           { "And this says".say; proceed } when * > 41;
           "This never says".say;
        }
        "This also says".say;
    }
=head2 X<given as a statement|control flow,given statement>

C<given> can follow a statement to set the topic in the statement it follows.

    .say given "foo";
    # OUTPUT: «foo␤»

    printf "%s %02i.%02i.%i",
            <Mo Tu We Th Fr Sa Su>[.day-of-week - 1],
            .day,
            .month,
            .year
        given DateTime.now;
    # OUTPUT: «Sa 03.06.2016»

=head1 X<loop|control flow, loop>

The C<loop> statement takes three statements in parentheses separated by C<;>
that take the role of initializer, conditional and incrementer. The initializer
is executed once and any variable declaration will spill into the surrounding
block. The conditional is executed once per iteration and coerced to C<Bool>,
if C<False> the loop is stopped. The incrementer is executed once per
iteration.

    loop (my $i = 0; $i < 10; $i++) {
        say $i;
    }

The infinite loop does not require parentheses.

=for code
loop { say 'forever' }

The C<loop> statement may be used to produce values from the result of each
run of the attached block if it appears in lists:

    (loop ( my $i = 0; $i++ < 3;) { $i * 2 }).say;               # OUTPUT: «(2 4 6)␤»
    my @a = (loop ( my $j = 0; $j++ < 3;) { $j * 2 }); @a.say;   # OUTPUT: «[2 4 6]␤»
    my @b = do loop ( my $k = 0; $k++ < 3;) { $k * 2 }; @b.say;  # same thing

Unlike a C<for> loop, one should not rely on whether returned values are
produced lazily.  It would probably be best to use C<eager> to guarantee that a
loop whose return value may be used actually runs:

    sub heads-in-a-row {
        (eager loop (; 2.rand < 1;) { "heads".say })
    }

=head1 X<while, until|control flow,while until>

The C<while> statement executes the block as long as its condition is
true. So

    my $x = 1;
    while $x < 4 {
        print $x++;
    }
    print "\n";

    # OUTPUT: «123␤»

Similarly, the C<until> statement executes the block as long as the
expression is false.

    my $x = 1;
    until $x > 3 {
        print $x++;
    }
    print "\n";

    # OUTPUT: «123␤»

The condition for C<while> or C<until> can be parenthesized, but there
must be a space between the keyword and the opening parenthesis of the
condition.

Both C<while> and C<until> can be used as statement modifiers. E. g.

   my $x = 42;
   $x-- while $x > 12

Also see C<repeat/while> and C<repeat/until> below.

All these forms may produce a return value the same way C<loop> does.

=head1 X<repeat/while, repeat/until|control flow,repeat>

Executes the block I<at least once> and, if the condition allows, repeats
that execution.  This differs from C<while>/C<until> in that the condition
is evaluated at the end of the loop, even if it appears at the front.

    my $x = -42;
    repeat {
        $x++;
    } while $x < 5;
    $x.say; # OUTPUT: «5␤»

    repeat {
        $x++;
    } while $x < 5;
    $x.say; # OUTPUT: «6␤»

    repeat while $x < 10 {
        $x++;
    }
    $x.say; # OUTPUT: «10␤»

    repeat while $x < 10 {
        $x++;
    }
    $x.say; # OUTPUT: «11␤»

    repeat {
        $x++;
    } until $x >= 15;
    $x.say; # OUTPUT: «15␤»

    repeat {
        $x++;
    } until $x >= 15;
    $x.say; # OUTPUT: «16␤»

    repeat until $x >= 20 {
        $x++;
    }
    $x.say; # OUTPUT: «20␤»

    repeat until $x >= 20 {
        $x++;
    }
    $x.say; # OUTPUT: «21␤»

All these forms may produce a return value the same way C<loop> does.

=head1 X<return|control flow,return>

The sub C<return> will stop execution of a subroutine or method, run all
relevant L<phasers|/language/phasers#Block_phasers> and provide the
given return value to the caller. The default return value is C<Nil>. If
a return L<type constraint|/type/Signature#Constraining_return_types> is
provided it will be checked unless the return value is C<Nil>. If the
type check fails the exception
L<X::TypeCheck::Return|/type/X::TypeCheck::Return> is thrown. If it
passes a control exception is raised and can be caught with
L<CONTROL|/language/phasers#CONTROL>.

Any C<return> in a block is tied to the first C<Routine> in the outer
lexical scope of that block, no matter how deeply nested.  Please note
that a C<return> in the root of a package will fail at runtime. A
C<return> in a block that is evaluated lazily (e.g. inside C<map>) may
find the outer lexical routine gone by the time the block is executed.
In almost any case C<last> is the better alternative. Please check
L<the functions documentation|/language/functions#Return_values> for
more information on how return values are handled and produced.


=head1 X<return-rw|control flow, return-rw>

The sub C<return> will return values, not containers. Those are
immutable and will lead to runtime errors when attempted to be mutated.

    sub s(){ my $a = 41; return $a };
    say ++s();
    CATCH { default { say .^name, ': ', .Str } };
    # OUTPUT: «X::Multi::NoMatch.new(dispatcher …

To return a mutable container, use C<return-rw>.

    sub s(){ my $a = 41; return-rw $a };
    say ++s();
    # OUTPUT: «42␤»

The same rules as for C<return> regarding phasers and control exceptions apply.

=head1 X<fail|control flow, fail>

Leaves the current routine and returns the provided
L<Exception|/type/Exception> or C<Str> wrapped inside a
L<Failure|/type/Failure>, after all relevant
L<phasers|/language/phasers#Block_phasers> are executed. If the caller
activated fatal exceptions via the pragma C<use fatal;>, the exception is
thrown instead of being returned as a C<Failure>.

    sub f { fail "WELP!" };
    say f;
    CATCH { default { say .^name, ': ', .Str } }
    # OUTPUT: «X::AdHoc: WELP!␤»

=head1 X<once|control flow, once>

A block prefix with C<once> will be executed exactly once, even if placed inside a
loop or a recursive routine.

    my $guard = 3;
    loop {
        last if $guard-- <= 0;
        once { put 'once' };
        print 'many'
    } # OUTPUT: «once␤manymanymany»

This works per "clone" of the containing code object, so:

    ({ once 42.say } xx 3).map: {$_(), $_()}; # says 42 thrice

Note that this is B<not> a thread-safe construct when the same clone of the same
block is run by multiple threads.  Also remember that methods only have one
clone per class, not per object.

=head1 X<quietly|control flow, quietly>

A C<quietly> block will suppress all warnings generated in it.

    quietly { warn 'kaput!' };
    warn 'still kaput!';
    # OUTPUT: «still kaput! [...]␤»

Any warning generated from any routine called from within the block will also be
suppressed:

    sub told-you { warn 'hey...' };
    quietly { told-you; warn 'kaput!' };
    warn 'Only telling you now!'
    # OUTPUT: «Only telling you now!␤ [...] ␤»


=head1 LABELs

C<while>, C<until>, C<loop> and C<for> loops can all take a label, which can be
used to identify them for C<next>, C<last>, and C<redo>. Nested loops are
supported, for instance:

    OUTAHERE: while True  {
        for 1,2,3 -> $n {
            last OUTAHERE if $n == 2;
        }
    }

Labels can be used also within nested loops to name each loop, for instance:

=begin code
OUTAHERE:
loop ( my $i = 1; True; $i++ ) {
  OUTFOR:
    for 1,2,3 -> $n {
      # exits the for loop before its natural end
      last OUTFOR if $n == 2;
  }

  # exits the infinite loop
  last OUTAHERE if $i >= 2;
}
=end code

=head1 X<next|control flow, next>

The C<next> command starts the next iteration of the loop. So the code

=begin code

my @x = 1, 2, 3, 4, 5;
for @x -> $x {
    next if $x == 3;
    print $x;
}
=end code

prints "1245".

If the L<C<NEXT> phaser|/language/phasers#NEXT> is present, it runs before the next iteration:

=begin code
my Int $i = 0;
while ($i < 10) {
  if ($i % 2 == 0) {
    next;
  }

  say "$i is odd.";

  NEXT {
    $i++;
  }
}
# OUTPUT: «1 is odd.␤3 is odd.␤5 is odd.␤7 is odd.␤9 is odd.␤»
=end code

=head1 X<last|control flow, last>

The C<last> command immediately exits the loop in question.

=begin code

my @x = 1, 2, 3, 4, 5;
for @x -> $x {
    last if $x == 3;
    print $x;
}
=end code

prints "12".

If the L<C<LAST> phaser|/language/phasers#LAST> is present, it runs
before exiting the loop:

=begin code
my Int $i = 1;
while ($i < 10) {
  if ($i % 5 == 0) {
    last;
  }

  LAST {
    say "The last number was $i.";
  }
  NEXT {
    $i++;
  }
}
# OUTPUT: «The last number was 5.␤»
=end code

=head1 X<redo|control flow, redo>

The C<redo> command restarts the loop block without evaluating the
conditional again.

=for code
loop {
    my $x = prompt("Enter a number");
    redo unless $x ~~ /\d+/;
    last;
}

=end pod

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