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TITLE

Synopsis 4: Blocks and Statements

AUTHORS

    Larry Wall <larry@wall.org>

VERSION

    Created: 19 Aug 2004
    Last Modified: 18 Aug 2014
    Version: 135

This document summarizes Apocalypse 4, which covers the block and statement syntax of Perl.

The Relationship of Lexical and Dynamic Scopes

Control flow is a dynamic feature of all computer programming languages, but languages differ in the extent to which control flow is attached to declarative features of the language, which are often known as "static" or "lexical". We use the phrase "lexical scoping" in its industry-standard meaning to indicate those blocks that surround the current textual location. More abstractly, any declarations associated with those textual blocks are also considered to be part of the lexical scope, and this is where the term earns the "lexical" part of its name, in the sense that lexical scoping actually does define the "lexicon" for the current chunk of code, insofar as the definitions of variables and routines create a local domain-specific language.

We also use the term "dynamic scoping" in the standard fashion to indicate the nested call frames that are created and destroyed every time a function or method is called. In most interesting programs the dynamic scopes are nested quite differently from the lexical scopes, so it's important to distinguish carefully which kind of scoping we're talking about.

Further compounding the difficulty is that every dynamic scope's outer call frame is associated with a lexical scope somewhere, so you can't just consider one kind of scoping or the other in isolation. Many constructs define a particular interplay of lexical and dynamic features. For instance, unlike normal lexically scope variables, dynamic variables search up the dynamic call stack for a variable of a particular name, but at each "stop" along the way, they are actually looking in the lexical "pad" associated with that particular dynamic scope's call frame.

In Perl 6, control flow is designed to do what the user expects most of the time, but this implies that we must consider the declarative nature of labels and blocks and combine those with the dynamic nature of the call stack. For instance, a return statement always returns from the lexically scoped subroutine that surrounds it. But to do that, it may eventually have to peel back any number of layers of dynamic call frames internal to the subroutine's current call frame. The lexical scope supplies the declared target for the dynamic operation. There does not seem to be a prevailing term in the industry for this, so we've coined the term lexotic to refer to these strange operations that perform a dynamic operation with a lexical target in mind. Lexotic operators in Perl 6 include:

    return
    next
    last
    redo
    goto

Some of these operators also fall back to a purely dynamic interpretation if the lexotic interpretation doesn't work. For instance, next with a label will prefer to exit a loop lexotically, but if there is no loop with an appropriate label in the lexical context, it will then scan upward dynamically through the call frames for any loop with the appropriate label, even though that loop will not be lexically visible. (next without a label is purely dynamic.) Lexotic and dynamic control flow is implemented by a system of control exceptions. For the lexotic return of next, the control exception will contain the identity of the loop scope to be exited (since the label was already "used up" to discover that identity), but for the dynamic fallback, the exception will contain only the loop label to be matched dynamically. See "Control Exceptions" below.

The redo operator, as a variant of goto, directly transfers control to the first statement of the lexotically enclosed loop. Essentially, the compiler turns it into a goto with an implicitly generated (secret) label on that first statement. In order to know when that implicit label must be generated, we restrict redo to the current outer lexical scope. It may not be used dynamically. (If you find yourself wanting the dynamic variant, please use goto with an explicit label instead, so the compiler can know to pessimize any unrolling of that loop.)

The Relationship of Blocks and Declarations

Every block is a closure. (That is, in the abstract, they're all anonymous subroutines that take a snapshot of their lexical environment.) How a block is invoked and how its results are used are matters of context, but closures all work the same on the inside.

Blocks are delimited by curlies, or by the beginning and end of the current compilation unit (either the current file or the current EVAL string). Unlike in Perl 5, there are (by policy) no implicit blocks around standard control structures. (You could write a macro that violates this, but resist the urge.) Variables that mediate between an outer statement and an inner block (such as loop variables) should generally be declared as formal parameters to that block. There are three ways to declare formal parameters to a closure.

    $func = sub ($a, $b) { .print if $a eq $b };  # standard sub declaration
    $func = -> $a, $b { .print if $a eq $b };     # a "pointy" block
    $func = { .print if $^a eq $^b }              # placeholder arguments

A bare closure (except the block associated with a conditional statement) without placeholder arguments that uses $_ (either explicitly or implicitly) is treated as though $_ were a formal parameter:

    $func = { .print if $_ };   # Same as: $func = <-> $_ { .print if $_ };
    $func("printme");

In any case, all formal parameters are the equivalent of my variables within the block. See S06 for more on function parameters.

Except for such formal parameter declarations, all lexically scoped declarations are visible from the point of declaration to the end of the enclosing block. Period. Lexicals may not "leak" from a block to any other external scope (at least, not without some explicit aliasing action on the part of the block, such as exportation of a symbol from a module). The "point of declaration" is the moment the compiler sees "my $foo", not the end of the statement as in Perl 5, so

    my $x = $x;

will no longer see the value of the outer $x; you'll need to say either

    my $x = $OUTER::x;

or

    my $x = OUTER::<$x>;

instead.

If you declare a lexical twice in the same scope, it is the same lexical:

    my $x;
    my $x;

By default the second declaration will get a compiler warning. You may suppress this by modifying the first declaration with proto:

    my proto $x;
    ...
    while my $x = @x.shift {...}              # no warning
    while my $x = @x.shift {...}              # no warning

If you've referred to $x prior to the first declaration, and the compiler tentatively bound it to $OUTER::x, then it's an error to declare it, and the compiler is required to complain at that point. If such use can't be detected because it is hidden in an EVAL, then it is erroneous, since the EVAL() compiler might bind to either $OUTER::x or the subsequently declared "my $x".

As in Perl 5, "our $foo" introduces a lexically scoped alias for a variable in the current package.

The new constant declarator introduces a compile-time constant, either a variable or named value, which may be initialized with a pseudo-assignment:

    constant $pi of Int = 3;
    my Num constant π = atan2(2,2) * 4;

The initializing expression is evaluated at BEGIN time. Constants (and enums) default to our scoping so they can be accessed from outside the package.

There is a new state declarator that introduces a lexically scoped variable like my does, but with a lifetime that persists for the life of the closure, so that it keeps its value from the end of one call to the beginning of the next. Separate clones of the closure get separate state variables. However, recursive calls to the same clone use the same state variable.

Perl 5's "local" function has been renamed to temp to better reflect what it does. There is also a let prefix operator that sets a hypothetical value. It works exactly like temp, except that the value will be restored only if the current block exits unsuccessfully. (See Definition of Success below for more.) temp and let temporize or hypotheticalize the value or the variable depending on whether you do assignment or binding. One other difference from Perl 5 is that the default is not to undefine a variable. So

    temp $x;

causes $x to start with its current value. Use

    undefine temp $x;

to get the Perl 5 behavior.

Note that temporizations that are undone upon scope exit must be prepared to be redone if a continuation within that scope is taken.

The Relationship of Blocks and Statements

In the absence of explicit control flow terminating the block early, the return value of a block is the value of its final statement. This is defined as the textually last statement of its top-level list of statements; any statements embedded within those top-level statements are in their own lower-level list of statements and, while they may be a final statement in their subscope, they're not considered the final statement of the outer block in question.

This is subtly different from Perl 5's behavior, which was to return the value of the last expression evaluated, even if that expression was just a conditional. Unlike in Perl 5, if a final statement in Perl 6 is a conditional that does not execute any of its branches, it doesn't matter what the value of the conditional is, the value of that conditional statement is always (). If there are no statements in the block at all, the result is also ().

Statement-ending blocks

A line ending with a closing brace "}", followed by nothing but whitespace or comments, will terminate a statement if an end of statement can occur there. That is, these two statements are equivalent:

    my $x = sub { 3 }
    my $x = sub { 3 };

Since bracketed expressions consider their insides to be statements, this works out consistently even where you might expect problems:

    my $x = [
        sub { 3 },  # this comma is not optional
        sub { 3 }   # the statement inside [] terminates here
    ];
    my $hash = {
        1 => { 2 => 3, 4 => 5 },  # OK
        2 => { 6 => 7, 8 => 9 }   # OK, terminates inner statement
    };

Because subroutine declarations are expressions, not statements, this is now invalid:

    sub f { 3 } sub g { 3 }     # two terms occur in a row

But these two are valid:

    sub f { 3 }; sub g { 3 };
    sub f { 3 }; sub g { 3 }    # the trailing semicolon is optional

Though certain control statements could conceivably be parsed in a self-contained way, for visual consistency all statement-terminating blocks that end in the middle of a line must be terminated by semicolon unless they are naturally terminated by some other statement terminator:

    while yin() { yang() }  say "done";      # ILLEGAL
    while yin() { yang() }; say "done";      # okay, explicit semicolon
    @yy := [ while yin() { yang() } ];       # okay within outer [...]
    while yin() { yang() } ==> sort          # okay, ==> separates statements

Conditional statements

The if and unless statements work much as they do in Perl 5. However, you may omit the parentheses on the conditional:

    if $foo == 123 {
        ...
    }
    elsif $foo == 321 {
        ...
    }
    else {
        ...
    }

The result of a conditional statement is the result of the block chosen to execute. If the conditional does not execute any branch, the return value is ().

The unless statement does not allow an elsif or else in Perl 6.

The value of the conditional expression may be optionally bound to a closure parameter:

    if    testa() -> $a { say $a }
    elsif testb() -> $b { say $b }
    else          -> $b { say $b }

Note that the value being evaluated for truth and subsequently bound is not necessarily a value of type Bool. (All normal types in Perl may be evaluated for truth. In fact, this construct would be relatively useless if you could bind only boolean values as parameters, since within the closure you already know whether it evaluated to true or false.) Binding within an else automatically binds the value tested by the previous if or elsif, which, while known to be false, might nevertheless be an interesting value of false. (By similar reasoning, an unless allows binding of a false parameter.)

An explicit placeholder may also be used:

    if blahblah() { return $^it }

However, use of $_ with a conditional or conditionally repeating statement's block is not considered sufficiently explicit to turn a 0-ary block into a 1-ary function, so all these methods use the same invocant:

    if .haste { .waste }
    while .haste { .waste }

(Contrast with a non-conditional statement such as:

    for .haste { .waste }

where each call to the block would bind a new invocant for the .waste method, each of which is likely different from the original invocant to the .haste method.)

Conditional statement modifiers work as in Perl 5. So do the implicit conditionals implied by short-circuit operators. Note though that the contents of parens or brackets is parsed as a statement, so you can say:

    @x = 41, (42 if $answer), 43;

and that is equivalent to:

    @x = 41, ($answer ?? 42 !! ()), 43

(Only a single statement is allowed inside parens or brackets; otherwise it will be interpreted as a LoL composer. See "Multidimensional slices and parcels" in S02.)

Loop statements

Looping statement modifiers are the same as in Perl 5 except that, for ease of writing list comprehensions, a looping statement modifier is allowed to contain a single conditional statement modifier:

    @evens = ($_ * 2 if .odd for 0..100);

Loop modifiers next, last, and redo also work much as in Perl 5. However, the labeled forms can use method call syntax: LABEL.next, etc. The .next and .last methods take an optional argument giving the final value of that loop iteration. So the old next LINE syntax is still allowed but really does something like LINE.next(()) underneath. Any block object can be used, not just labels, so to return a value from this iteration of the current block you can say:

    &?BLOCK.next($retval);

[Conjecture: a bare next($retval) function could be taught to do the same, as long as $retval isn't a loop label. Presumably multiple dispatch could sort this out.]

With a target object or label, loop modifiers search lexotically for the scope to modify. Without a target, however, they are purely dynamic, and choose the innermost dynamic loop, which may well be a map or other implicitly looping function, including user-defined functions.

There is no longer a continue block. Instead, use a NEXT block within the body of the loop. See below.

The value of a loop statement is the list of values from each iteration. Each iteration's value is returned as a single "argument" object. See S02 for a long definition of argument, but in short, it's either an ordinary object or a parcel containing multiple values.

Normal flat list context ignores parcel boundaries and flattens the list.

Iterations that return () (such as by calling next with no extra return arguments) return that () as the next value, which will therefore disappear when interpolated in flat context, but will interpolate an empty Parcel into slice context.

For finer-grained control of which iterations return values, use gather and take.

The while and until statements

The while and until statements work as in Perl 5, except that you may leave out the parentheses around the conditional:

    while $bar < 100 {
        ...
    }

As with conditionals, you may optionally bind the result of the conditional expression to a parameter of the block:

    while something() -> $thing {
        ...
    }
    while something() { ... $^thing ... }

Nothing is ever bound implicitly, however, and many conditionals would simply bind True or False in an uninteresting fashion. This mechanism is really only good for objects that know how to return a boolean value and still remain themselves. In general, for most iterated solutions you should consider using a for loop instead (see below). In particular, we now generally use for to iterate filehandles.

The repeat statement

Unlike in Perl 5, applying a statement modifier to a do block is specifically disallowed:

    do {
        ...
    } while $x < 10;    # ILLEGAL

Instead, you should write the more Pascal-like repeat loop:

    repeat {
        ...
    } while $x < 10;

or equivalently:

    repeat {
        ...
    } until $x >= 10;

Unlike Perl 5's do-while loop, this is a real loop block now, so next, last, and redo work as expected. The loop conditional on a repeat block is required, so it will be recognized even if you put it on a line by its own:

    repeat
    {
        ...
    }
    while $x < 10;

However, that's likely to be visually confused with a following while loop at the best of times, so it's also allowed to put the loop conditional at the front, with the same meaning. (The repeat keyword forces the conditional to be evaluated at the end of the loop, so it's still C's do-while semantics.) Therefore, even under GNU style rules, the previous example may be rewritten into a very clear:

    repeat while $x < 10
      {
        ...
      }

or equivalently:

    repeat until $x >= 10
      {
        ...
      }

As with an ordinary while, you may optionally bind the result of the conditional expression to a parameter of the block:

    repeat -> $thing {
        ...
    } while something();

or

    repeat while something() -> $thing {
        ...
    }

Since the loop executes once before evaluating the condition, the bound parameter will be undefined that first time through the loop.

The general loop statement

The loop statement is the C-style for loop in disguise:

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

As in C, the parentheses are required if you supply the 3-part spec; however, the 3-part loop spec may be entirely omitted to write an infinite loop. That is,

    loop {...}

is equivalent to the Cish idiom:

    loop (;;) {...}

The for statement

There is no foreach statement any more. It's always spelled for in Perl 6, so it always takes a list as an argument:

    for @foo { .print }

As mentioned earlier, the loop variable is named by passing a parameter to the closure:

    for @foo -> $item { print $item }

Multiple parameters may be passed, in which case the list is traversed more than one element at a time:

    for %hash.kv -> $key, $value { print "$key => $value\n" }

To process two arrays in parallel use the zip function to generate a list that can be bound to the corresponding number of parameters:

    for zip(@a;@b) -> $a, $b { print "[$a, $b]\n" }
    for @a Z @b -> $a, $b { print "[$a, $b]\n" }        # same thing

The list is evaluated lazily by default, so instead of using a while to read a file a line at a time as you would in Perl 5:

    while (my $line = <STDIN>) {...}

in Perl 6 you should use a for instead:

    for $*IN.lines -> $line {...}

This has the added benefit of limiting the scope of the $line parameter to the block it's bound to. (The while's declaration of $line continues to be visible past the end of the block. Remember, no implicit block scopes.) It is also possible to write

    while $*IN.get -> $line {...}

However, this is likely to fail on autochomped filehandles, so use the for loop instead.

Note also that Perl 5's special rule causing

    while (<>) {...}

to automatically assign to $_ is not carried over to Perl 6. That should now be written:

    for lines() {...}

which is short for

    for lines($*ARGFILES) {...}

Arguments bound to the formal parameters of a pointy block are by default readonly within the block. You can declare a parameter read/write by including the "is rw" trait. The following treats every other value in @values as modifiable:

    for @values -> $even is rw, $odd { ... }

In the case where you want all your parameters to default to rw, you may use the visually suggestive double-ended arrow to indicate that values flow both ways:

    for @values <-> $even, $odd { ... }

This is equivalent to

    for @values -> $even is rw, $odd is rw { ... }

If you rely on $_ as the implicit parameter to a block, then $_ is considered read/write by default. That is, the construct:

    for @foo {...}

is actually short for:

    for @foo <-> $_ {...}

so you can modify the current list element in that case.

When used as statement modifiers on implicit blocks (thunks), for and given privately temporize the current value of $_ for the left side of the statement and restore the original value at loop exit:

    $_ = 42;
    .say             # 42
    .say for 1,2,3;  # 1,2,3
    .say;            # 42

The previous value of $_ is not available within the loop. If you want it to be available, you must rewrite it as an explicit block using curlies:

    { say OUTER::<$_>, $_ } for 1,2,3;  # 421,422,423

No temporization is necessary with the explicit form since $_ is a formal parameter to the block. Likewise, temporization is never needed for statement_control:<for> because it always calls a closure.

The do-once loop

In Perl 5, a bare block is deemed to be a do-once loop. In Perl 6, the bare block is not a do-once. Instead do {...} is the do-once loop (which is another reason you can't put a statement modifier on it; use repeat for a test-at-the-end loop).

For any statement, prefixing with a do allows you to return the value of that statement and use it in an expression:

    $x = do if $a { $b } else { $c };

This construct only allows you to attach a single statement to the end of an expression. If you want to continue the expression after the statement, or if you want to attach multiple statements, you must either use the curly form or surround the entire expression in brackets of some sort:

    @primesquares = (do $_ if .is-prime for 1..100) »**» 2;

Since a bare expression may be used as a statement, you may use do on an expression, but its only effect is to function as an unmatched left parenthesis, much like the $ operator in Haskell. That is, precedence decisions do not cross a do boundary, and the missing "right paren" is assumed at the next statement terminator or unmatched bracket. A do is unnecessary immediately after any opening bracket as the syntax inside brackets expects a statements, so the above can in fact be written:

    @primesquares = ($_ if .is-prime for 1..100) »**» 2;

This basically gives us list comprehensions as rvalue expressions:

    (for 1..100 { $_ if .is-prime }).say

Another consequence of this is that any block just inside a left parenthesis is immediately called like a bare block, so a multidimensional list comprehension may be written using a block with multiple parameters fed by a for modifier:

    @names = (-> $name, $num { "$name.$num" } for 'a'..'zzz' X 1..100);

or equivalently, using placeholders:

    @names = ({ "$^name.$^num" } for 'a'..'zzz' X 1..100);

Since do is defined as going in front of a statement, it follows that it can always be followed by a statement label. This is particularly useful for the do-once block, since it is officially a loop and can take therefore loop control statements.

Loops at the statementlist level vs the statement level

In any sequence of statements, only the value of the final statement is returned, so all prior statements are evaluated in sink context, which is automatically eager, to force the evaluation of side effects. (Side effects are the only reason to execute such statements in the first place, and Perl will, in fact, warn you if you do something that is "useless" in sink context.) A loop in sink context not only evaluates itself eagerly, but can optimize away the production of any values from the loop.

The final statement of a statement list is not a sink context, and can return any value including a lazy list. However, to support the expectations of imperative programmers (the vast majority of us, it turns out), any explicit loop found as the final statement of a statement list is automatically forced to use sink semantics so that the loop executes to completion before returning from the block.

This forced sink context is applied to loops only at the statement list level, that is, at the top level of a compilation unit, or directly inside a block. Constructs that parse a single statement or semilist as an argument are presumed to want the results of that statement, so such constructs remain lazy even when that statement is a loop. Assuming each of the following statements is the final statement in a block, "sunk" loops such as these may be indicated:

    for LIST { ... }
    ... if COND for LIST 
    loop { ... }
    ... while COND
    while COND { ... }
    repeat until COND { ... }

but lazy loops can be indicated by putting the loop in parens or brackets:

    (... if COND for LIST)      # lazy list comprehension
    [for LIST { ... }] 
    (loop { ... })

or by use of either a statement prefix or a phaser in statement form:

    lazy for LIST { ... }
    ENTER for LIST { ... }

Note that the corresponding block forms put the loop into a statement list, so these loops are evaluated in sink context:

    lazy { for LIST { ... } }   # useless use of 'lazy' here
    ENTER { for LIST { ... } }

It doesn't matter that there is only one statement there; what matters is that a sequence of statements is expected there by the grammar.

An eager loop may likewise be indicated by using the eager statement prefix:

    eager for LIST { ... }
    eager ... if COND for LIST 
    eager loop { ... }
    eager ... while COND
    eager while COND { ... }
    eager repeat until COND { ... }

It is erroneous to write an eager loop without a loop exit, since that will chew up all your memory.

Note that since do is considered a one-time loop, it is always evaluated eagerly, despite being a statement prefix. This is no great hardship; the lazy prefix is better documentation in any case. And surely the verb "do" ought to imply some degree of getting it done eagerly.

The given construct is not considered a loop, and just returns normally.

Statement-level bare blocks

Although a bare block occurring as a single statement is no longer a do-once loop, as with loops when used in a statement list, it still executes immediately as in Perl 5, as if it were immediately dereferenced with a .() postfix, so within such a block CALLER:: refers to the dynamic scope associated with the lexical scope surrounding the block.

If you wish to return a closure from a function, you must use an explicit prefix such as return or sub or ->.

    sub f1
    {
        # lots of stuff ...
        { say "I'm a closure." }
    }
    my $x1= f1;  # fall-off return is result of the say, not the closure.
    sub f2
    {
        # lots of stuff ...
        return { say "I'm a closure." }
    }
    my $x2= f2;  # returns a Block object.

Use of a placeholder parameter in statement-level blocks triggers a syntax error, because the parameter is not out front where it can be seen. However, it's not an error when prefixed by a do, or when followed by a statement modifier:

    # Syntax error: Statement-level placeholder block
    { say $^x };
    # Not a syntax error, though $x doesn't get the argument it wants
    do { say $^x };
    # Not an error: Equivalent to "for 1..10 -> $x { say $x }"
    { say $^x } for 1..10;
    # Not an error: Equivalent to "if foo() -> $x { say $x }"
    { say $^x } if foo();

It's not an error to pass parameters to such a block either:

    { say $^x + $^x }(5);

But as always, you must use them all:

    # Syntax error: Too many positional parameters passed
    { say $^x + $^x }(5,6);

The gather statement prefix

A variant of do is gather. Like do, it is followed by a statement or block, and executes it once. Unlike do, it evaluates the statement or block in sink (void) context; its return value is instead specified by calling the take list prefix operator one or more times within the scope (either lexical or dynamic) of the gather. The take function's signature is like that of return; while having the syntax of a list operator, it merely returns a single item or "argument" (see S02 for definition).

The take function is lexotic if there is a visible outer gather, but falls back to purely dynamic if not. Well, it doesn't really fall back, since a take knows at compile time whether it is being used lexically or dynamically. Less obviously, so does a gather; if a gather lexically contains any take calls, it is marked as lexotic-only, and it will be invisible to a dynamic take. If the gather contains no take lexically, it by definition cannot be the lexotic target of any take, so it can only harvest dynamic take calls. The only remaining difficulty arises if both the user and a library writer attempt to use dynamic gather with user-defined callbacks that contain take. So we will say that it is erroneous for a library writer to mix dynamic gather with callbacks unless those callbacks are somehow "ungathered" to the outer dynamic scope. [Conjecture: there should either be an callergather primitive that does this, or we should allow labeled gather/take for such a situation, and dynamic take must match the gather's label (or lack thereof) exactly. (Using the term "label" loosely, to include other solutions besides the label syntax, such as .gather and .take methods on some identity object.)]

If you take multiple items in a comma list (since it is, after all, a list operator), they will be wrapped up in a Parcel object for return as the next argument. No additional context is applied by the take operator, since all context is lazy in Perl 6. The flattening or slicing of any such returned parcel will be dependent on how the gather's return iterator is iterated (with .get vs .getarg).

The value returned by the take to the take's own context is that same returned argument (which is ignored when the take is in sink context). Regardless of the take's immediate context, the object returned is also added to the list of values being gathered, which is returned by the gather as a lazy list (that is, an iterator, really), with each argument element of that list corresponding to one take.

Any parcels in the returned list are normally flattened when bound into flat context. When bound into a lol context, however, the parcel objects become real List objects that keep their identity as discrete sublists. The eventual binding context thus determines whether to throw away or keep the groupings resulting from each individual take call. Most list contexts are flat rather than sliced, so the boundaries between individual take calls usually disappear. (FLAT is an acronym meaning Flat Lists Are Typical. :)

Because gather evaluates its block or statement in sink context, this typically causes the take function to be evaluated in sink context. However, a take function that is not in sink context gathers its return objects en passant and also returns them unchanged. This makes it easy to keep track of what you last "took":

    my @uniq = gather for @list {
        state $previous = take $_;
        next if $_ === $previous;
        $previous = take $_;
    }

The take function essentially has two contexts simultaneously, the context in which the gather is operating, and the context in which the take is operating. These need not be identical contexts, since they may bind or coerce the resulting parcels differently:

    my @y;
    my @x = gather for 1..2 {            # flat context for list of parcels
        my ($y) := \(take $_, $_ * 10);  # binding forces item context
        push @y, $y;
    }
    # @x contains 4 Ints:    1,10,2,20 flattened by list assignment to @x
    # @y contains 2 Parcels: $(1,10),$(2,20) sliced by binding to positional $y

Likewise, we can just remember the gather's result parcel by binding and later coercing it:

    my ($c) := \(gather for 1..2 {
        take $_, $_ * 10;
    });
    # $c.flat produces 1,10,2,20 -- flatten fully into a list of Ints.
    # $c.lol produces LoL.new($(1,10),$(2,20)) -- list of Parcels, a 2-D list.
    # $c.item produces ($(1,10),$(2,20)).list.item -- a list of Parcels, as an item.

Note that the take itself is in sink context in this example because the for loop is in the sink context provided inside the gather.

A gather is not considered a loop, but it is easy to combine with a loop statement as in the examples above.

The take operation may be defined internally using resumable control exceptions, or dynamic variables, or pigeons carrying clay tablets. The choice any particular implementation makes is specifically not part of the definition of Perl 6, and you should not rely on it in portable code.

Other do-like forms

Other similar forms, where a keyword is followed by code to be controlled by it, may also take bare statements, including try, once, quietly, start, lazy, and sink. These constructs establish a dynamic scope without necessarily establishing a lexical scope. (You can always establish a lexical scope explicitly by using the block form of argument.) As statement introducers, all these keywords must be followed by whitespace. (You can say something like try({...}), but then you are calling the try() function using function call syntax instead, and since Perl does not supply such a function, it will be assumed to be a user-defined function.) For purposes of flow control, none of these forms are considered loops, but they may easily be applied to a normal loop.

Note that any construct in the statement_prefix category defines special syntax. If followed by a block it does not parse as a list operator or even as a prefix unary; it will never look for any additional expression following the block. In particular,

    foo( try {...}, 2, 3 )

calls the foo function with three arguments. And

    do {...} + 1

add 1 to the result of the do block. On the other hand, if a statement_prefix is followed by a non-block statement, all nested blockless statement_prefixes will terminate at the same statement ending:

    do do do foo(); bar 43;

is parsed as:

    do { do { do { foo(); }}}; bar(43);

Switch statements

A switch statement is a means of topicalizing, so the switch keyword is the English topicalizer, given. The keyword for individual cases is when:

    given EXPR {
        when EXPR { ... }
        when EXPR { ... }
        default { ... }
    }

The current topic is always aliased to the special variable $_. The given block is just one way to set the current topic, but a switch statement can be any block that sets $_, including a for loop (assuming one of its loop variables is bound to $_) or the body of a method (if you have declared the invocant as $_). So switching behavior is actually caused by the when statements in the block, not by the nature of the block itself. A when statement implicitly does a "smart match" between the current topic ($_) and the argument of the when. If the smart match succeeds, when's associated block is executed, and the innermost surrounding block that has $_ as one of its formal parameters (either explicit or implicit) is automatically broken out of. (If that is not the block you wish to leave, you must use the LABEL.leave method (or some other control exception such as return or next) to be more specific, since the compiler may find it difficult to guess which surrounding construct was intended as the actual topicalizer.) The value of the inner block is returned as the value of the outer block.

If the smart match fails, control proceeds the next statement normally, which may or may not be a when statement. Since when statements are presumed to be executed in order like normal statements, it's not required that all the statements in a switch block be when statements (though it helps the optimizer to have a sequence of contiguous when statements, because then it can arrange to jump directly to the first appropriate test that might possibly match.)

The default case:

    default {...}

is exactly equivalent to

    when * {...}

Because when statements are executed in order, the default must come last. You don't have to use an explicit default--you can just fall off the last when into ordinary code. But use of a default block is good documentation.

If you use a for loop with a parameter named $_ (either explicitly or implicitly), that parameter can function as the topic of any when statements within the loop.

You can explicitly break out of a when block (and its surrounding topicalizer block) early using the succeed verb. More precisely, it first scans outward (lexically) for the innermost containing when block. From there it continues to scan outward to find the innermost block outside the when that defines $_, either explicitly or implicitly. (Note that both of these scans are done at compile time; if the scans fail, it's a compile-time semantic error.) Typically, such an outer block will be the block of a given or a for statement, but any block that sets the topic can be broken out of. At run time, succeed uses a control exception to scan up the dynamic chain to find the call frame belonging to that same outer block, and when it has found that frame, it does a .leave on it to unwind the call frames. If any arguments are supplied to the succeed function, they are passed out via the leave method. Since leaving a block is considered a successful return, breaking out of one with succeed is also considered a successful return for the purposes of KEEP and UNDO.

The implicit break of a normal when block works the same way, returning the value of the entire block (normally from its last statement) via an implicit succeed.

You can explicitly leave a when block and go to the next statement following the when by using proceed. (Note that, unlike C's idea of "falling through", subsequent when conditions are evaluated. To jump into the next when block without testing its condition, you must use a goto. But generally that means you should refactor instead.)

If you have a switch that is the main block of a for loop that uses $_ as its loop variable, and you break out of the switch either implicitly or explicitly (that is, the switch "succeeds"), control merely goes to the end of that block, and thence on to the next iteration of the loop. You must use last (or some more violent control exception such as return) to break out of the entire loop early. Of course, an explicit next might be clearer than a succeed if you really want to go directly to the next iteration. On the other hand, succeed can take an optional argument giving the value for that iteration of the loop. As with the .leave method, there is also a .succeed method to break from a labelled block functioning as a switch:

    OUTER.succeed($retval)

There is a when statement modifier, but it does not have any breakout semantics; it is merely a smartmatch against the current topic. That is,

    doit() when 42;

is exactly equivalent to

    doit() if $_ ~~ 42;

This is particularly useful for list comprehensions:

    @lucky = ($_ when /7/ for 1..100);

Exception handlers

Unlike many other languages, Perl 6 specifies exception handlers by placing a CATCH block within that block that is having its exceptions handled.

The Perl 6 equivalent to Perl 5's eval {...} is try {...}. (Perl 6's EVAL function only evaluates strings, not blocks, and does not catch exceptions.) A try block by default has a CATCH block that handles all fatal exceptions by ignoring them. If you define a CATCH block within the try, it replaces the default CATCH. It also makes the try keyword redundant, because any block can function as a try block if you put a CATCH block within it. To prevent lazy lists for leaking out unexpectedly, the inside of a try is always considered an eager context, unless the try itself is in a sink context, in which case the inside of try is also in sink context.

Additionally, the try block or statement implicitly enforces a use fatal context such that failures are immediately thrown as exceptions. (See below.)

An exception handler is just a switch statement on an implicit topic that happens to be the current exception to be dealt with. Inside the CATCH block, the exception in question is bound to $_. Because of smart matching, ordinary when statements are sufficiently powerful to pattern match the current exception against classes or patterns or numbers without any special syntax for exception handlers. If none of the cases in the CATCH handles the exception, the exception will be rethrown. To ignore all unhandled exceptions, use an empty default case. (In other words, there is an implicit .die just inside the end of the CATCH block. Handled exceptions break out past this implicit rethrow.) Hence, CATCH is unlike all other switch statements in that it treats code inside a default block differently from code that's after all the when blocks but not in a default block.

More specifically, when you write:

    CATCH {
        when Mumble {...}
        default {...}
    }

you're really calling into a catch lambda that works something like this:

    -> *@! {
        my @handled = ();
        my @unhandled = ();
        my @*undead = ();
        for @! {
            # note, fails current iteration, continues with loop
            SIMPLECATCH { push @*undead, $_; push @unhandled, OUTER::<$_>; }
            .handled = True;
            when Mumble {...}
            default {...}
            .handled = False;
            push @unhandled, $_;
            KEEP { push @handled, $_ if .handled }
        }
        push @unhandled, @*undead;
        # no point in setting their $! if we're gonna blow past
        set_outer_caller's_bang(@handled) unless @unhandled;
        @unhandled;
    }

Whenever an exception occurs during the execution of a handler, it is pushed onto the end of the @*undead array for later processing by an outer handler. If there are any unhandled @! exceptions, or if any exceptions were caught by the inner SIMPLECATCH (which does nothing but runs its push code, which should not produce any exceptions), then the CATCH block returns them to the exception thrower.

The exception thrower looks up the call stack for a catch lambda that returns () to indicate all exceptions are handled, and then it is happy, and unwinds the stack to that point. If any exceptions are returned as not handled, the exception thrower keeps looking for a higher dynamic scope for a spot to unwind to. Note that any die in the catch lambda eventually rethrows outside the lambda as a new exception, but not until the current exception handler has a chance to handle all exceptions that came in via @!.

Resumable exceptions may or may not leave normally depending on the implementation. If continuations are used, the .resume call will simply goto the continuation in question, and the lambda's callframe is abandoned. Resumable exceptions may also be implemented by simply marking the current exception as "resumed", in which case the original exception thrower simply returns to the code that threw the resumable exception, rather than unwinding before returning. This could be done by pushing the resumed exception onto the unhandled list, and then the thrower checking to see if there is only a single resumed exception in the "unhandled" list. The unhandled list is a dynamic variable so that it's easy for .resume to manipulate it.

A CATCH block sees the lexical scope in which it was defined, but its caller is the dynamic location that threw the exception. That is, the stack is not unwound until some exception handler chooses to unwind it by "handling" the exception in question. So logically, if the CATCH block throws its own exception, you would expect the CATCH block to catch its own exception recursively forever. However, a CATCH must not behave that way, so we say that a CATCH block never attempts to handle any exception thrown within its own dynamic scope. (Otherwise any die would cause an infinite loop.) Instead we treasure them up and rethrow them to a handler further up.

Unlike try, the presence of a CATCH block does not imply use fatal semantics for failures; you may, however, use either an explicit try block around the CATCH or an explicit use fatal to guarantee that failures are thrown eagerly rather than lazily.

Control Exceptions

All abnormal control flow is, in the general case, handled by the exception mechanism (which is likely to be optimized away in specific cases.) Here "abnormal" means any transfer of control outward that is not just falling off the end of a block. A return, for example, is considered a form of abnormal control flow, since it can jump out of multiple levels of closures to the end of the scope of the current subroutine definition. Loop commands like next are abnormal, but looping because you hit the end of the block is not. The implicit break (what succeed does explicitly) of a when block is abnormal.

A CATCH block handles only "bad" exceptions, and lets control exceptions pass unhindered. Control exceptions may be caught with a CONTROL block. Generally you don't need to worry about this unless you're defining a control construct. You may have one CATCH block and one CONTROL block, since some user-defined constructs may wish to supply an implicit CONTROL block to your closure, but let you define your own CATCH block.

A return always exits from the lexically surrounding sub or method definition (that is, from a function officially declared with the sub, method, or submethod keywords). Pointy blocks and bare closures are transparent to return, in that the return statement still means &?ROUTINE.leave from the Routine that existed in dynamic scope when the closure was cloned.

It is illegal to return from the closure if that Routine no longer owns a call frame in the current call stack.

To return a value (to the dynamical caller) from any pointy block or bare closure, you either just let the block return the value of its final expression, or you can use leave, which comes in both function and method forms. The function (or listop) form always exits from the innermost block, returning its arguments as the final value of the block exactly as return does. The method form will leave any block in the dynamic scope that can be named as an object and that responds to the .leave method.

Hence, the leave function:

    leave(1,2,3)

is really just short for:

    &?BLOCK.leave(1,2,3)

To return from your immediate caller, you can say:

    caller.leave(1,2,3)

Further call frames up the caller stack may be located by use of the callframe function:

    callframe({ .labels.any eq 'LINE' }).leave(1,2,3);

By default the innermost call frame matching the selection criteria will be exited. This can be a bit cumbersome, so in the particular case of labels, the label that is already visible in the current lexical scope is considered a kind of pseudo object specifying a potential dynamic context. If instead of the above you say:

    LINE.leave(1,2,3)

it was always exit from your lexically scoped LINE loop, even if some inner dynamic scope you can't see happens to also have that label. (In other words, it's lexotic.) If the LINE label is visible but you aren't actually in a dynamic scope controlled by that label, an exception is thrown. (If the LINE is not visible, it would have been caught earlier at compile time since LINE would likely be a bareword.)

In theory, any user-defined control construct can catch any control exception it likes. However, there have to be some culturally enforced standards on which constructs capture which exceptions. Much like return may only return from an "official" subroutine or method, a loop exit like next should be caught by the construct the user expects it to be caught by. (Always assuming the user expects the right thing, of course...) In particular, if the user labels a loop with a specific label, and calls a loop control from within the lexical scope of that loop, and if that call mentions the outer loop's label, then that outer loop is the one that must be controlled. In other words, it first tries this form:

    LINE.leave(1,2,3)

If there is no such lexically scoped outer loop in the current subroutine, then a fallback search is made outward through the dynamic scopes in the same way Perl 5 does. (The difference between Perl 5 and Perl 6 in this respect arises only because Perl 5 didn't have user-defined control structures, hence the sub's lexical scope was always the innermost dynamic scope, so the preference to the lexical scope in the current sub was implicit. For Perl 6 we have to make this preference for lexotic behavior explicit.)

Warnings are produced in Perl 6 by throwing a resumable control exception to the outermost scope, which by default prints the warning and resumes the exception by extracting a resume continuation from the exception, which must be supplied by the warn() function (or equivalent). Exceptions are not resumable in Perl 6 unless the exception object does the Resumable role. (Note that fatal exception types can do the Resumable role even if thrown via fail()--when uncaught they just hit the outermost fatal handler instead of the outermost warning handler, so some inner scope has to explicitly treat them as warnings and resume them.)

Since warnings are processed using the standard control exception mechanism, they may be intercepted and either suppressed or fatalized anywhere within the dynamic scope by supplying a suitable CONTROL block. This dynamic control is orthogonal to any lexically scoped warning controls, which merely decide whether to call warn() in the first place.

As with calls to return, the warning control exception is an abstraction that the compiler is free to optimize away (along with the associated continuation) when the compiler or runtime can determine that the semantics would be preserved by merely printing out the error and going on. Since all exception handlers run in the dynamic scope of the throw, that reduces to simply returning from the warn function most of the time. See previous section for discussion of ways to return from catch lambdas. The control lambda is logically separate from the catch lambda, though an implementation is allowed to combine them if it is careful to retain separate semantics for catch and control exceptions.

One additional level of control is the notion of lazy warnings. If, instead of throwing a warning directly, the program calls fail() with a resumable exception, the throwing of the warning is delayed until first use (or the caller's policy) requires it to be thrown. If the warning exception supports the .resume_value method, that will be the value of the failure after it has resumed. Otherwise the value will be the null string. Numeric and string conversions use these lazy warnings to allow (but not require) failsoft semantics.

The goto statement

In addition to next, last, and redo, Perl 6 also supports goto. As with ordinary loop controls, the label is searched for first lexically within the current subroutine, then dynamically outside of it. Unlike with loop controls, however, scanning a scope includes a scan of any lexical scopes included within the current candidate scope. As in Perl 5, it is possible to goto into a lexical scope, but only for lexical scopes that require no special initialization of parameters. (Initialization of ordinary variables does not count--presumably the presence of a label will prevent code-movement optimizations past the label.) So, for instance, it's always possible to goto into the next case of a when or into either the "then" or "else" branch of a conditional. You may not go into a given or a for, though, because that would bypass a formal parameter binding (not to mention list generation in the case of for). (Note: the implicit default binding of an outer $_ to an inner $_ can be emulated for a bare block, so that doesn't fall under the prohibition on bypassing formal binding.)

Because it is possible to go to a label that is after the operation, and because Perl 6 does one-pass parsing, any goto to a label that has not been yet declared (or is declared outside the outward lexical scope of the goto) must enclose the label in quotes.

Exceptions

As in Perl 5, many built-in functions simply return an undefined value when you ask for a value out of range, or the function fails somehow. Perl 6 has Failure objects, known as "unthrown exceptions" (though really a Failure merely contains an unthrown exception), which know whether they have been handled or not. $! is a convenient link to the last failure, and only ever contains one exception, the most recent.

[Conjecture: all unhandled exceptions within a routine could be stored in @!, with the most recent first. $! would then be sugar for @![0]. (Or we use push semantics and $! means @![*-1].) This might be more robust than merely making @! a parameter to CATCH. However, the new semantics of autothrowing when sink eats a Failure means we won't have many unthrown exceptions waiting around to be handled at the end of the block anymore. We should probably at least issue warnings, though, if the GC eventually collects a failure that was never handled. We can't really rely on end-of-routine cleanup to deal with failures that are returned as normal data, unless we go with the overhead of a lexical @! variable.]

If you test a Failure for .defined or .Bool, the Failure marks itself as handled; the exception acts as a relatively harmless undefined value thereafter. Any other use of the Failure object to extract a normal value will throw its associated exception immediately. (The Failure may, however, be stored in any container whose type allows the Failure role to be mixed in.) The .handled method returns False on failures that have not been handled. It returns True for handled exceptions and for all non-Failure objects. (That is, it is a Mu method, not a Failure method. Only Failure objects need to store the actual status however; other types just return True.)

The .handled method is rw, so you may mark an exception as handled by assigning True to it. Note however that

    $!.handled = 1;

marks only the last exception as handled. To mark them all as handled you must access them individually via the implicit loop of a CATCH block.

A bare die/fail takes $! as the default argument specifying the exception to be thrown or propagated outward to the caller's $!.

You can cause built-ins to automatically throw exceptions on failure using

    use fatal;

The fail function responds to the caller's use fatal state. It either returns an unthrown exception, or throws the exception. Before you get too happy about this pragma, note that Perl 6 contains various parallel processing primitives that will tend to get blown up prematurely by thrown exceptions. Unthrown exceptions are meant to provide a failsoft mechanism in which failures can be treated as data and dealt with one by one, without aborting execution of what may be perfectly valid parallel computations. If you don't deal with the failures as data, then sink context will automatically throw any unhandled Failure that you try to discard.

In any case, the overriding design principle here is that no unhandled exception is ever dropped on the floor, but propagated outward until it is handled. If no explicit handler handles it, the implicit outermost exception handler will eventually decide to abort and print all unhandled exceptions passed in as its current @! list.

It is possible to fail with a resumable exception, such as a warning. If the failure throws its exception and the exception resumes, the thrower by default returns the null string ('') to whatever caused the failure to throw its exception. This may be overridden by attaching a .resume_value to the warning. Hence numeric coercions such as +"42foo" can be forced to return 42 after issuing a warning.

Phasers

A CATCH block is just a trait of the closure containing it, and is automatically called at the appropriate moment. These auto-called blocks are known as phasers, since they generally mark the transition from one phase of computing to another. For instance, a CHECK block is called at the end of compiling a compilation unit. Other kinds of phasers can be installed as well; these are automatically called at various times as appropriate, and some of them respond to various control exceptions and exit values. Phasers marked with a * can be used for their return value.

      BEGIN {...}*      at compile time, ASAP, only ever runs once
      CHECK {...}*      at compile time, ALAP, only ever runs once
       LINK {...}*      at link time, ALAP, only ever runs once
       INIT {...}*      at run time, ASAP, only ever runs once
        END {...}       at run time, ALAP, only ever runs once
      ENTER {...}*      at every block entry time, repeats on loop blocks.
      LEAVE {...}       at every block exit time (even stack unwinds from exceptions)
       KEEP {...}       at every successful block exit, part of LEAVE queue
       UNDO {...}       at every unsuccessful block exit, part of LEAVE queue
      FIRST {...}*      at loop initialization time, before any ENTER
       NEXT {...}       at loop continuation time, before any LEAVE
       LAST {...}       at loop termination time, after any LEAVE
        PRE {...}       assert precondition at every block entry, before ENTER
       POST {...}       assert postcondition at every block exit, after LEAVE
      CATCH {...}       catch exceptions, before LEAVE
    CONTROL {...}       catch control exceptions, before LEAVE
    COMPOSE {...}       when a role is composed into a class

Some of the statement prefixes also behave a little bit like phasers, but they run in-line with the executable code, so they are spelled in lowercase. They parse the same as phasers:

         do {...}*      run a block or statement as a term
       once {...}*      run only once, suppressing additional evaluations
     gather {...}*      start a co-routine thread
      eager {...}*      evaluate statement eagerly
       lazy {...}*      defer actual evaluation till value is fetched
       sink {...}*      evaluate eagerly but throw results away
        try {...}*      evaluate and trap exceptions (implies 'use fatal')
    quietly {...}*      evaluate and suppress warnings
      start {...}*      start computation of a promised result

Constructs marked with a * have a run-time value, and if evaluated earlier than their surrounding expression, they simply save their result for use in the expression later when the rest of the expression is evaluated:

    my $compiletime = BEGIN { now };
    our $temphandle = ENTER { maketemp() };

As with other statement prefixes, these value-producing constructs may be placed in front of either a block or a statement:

    my $compiletime = BEGIN now;
    our $temphandle = ENTER maketemp();

In fact, most of these phasers will take either a block or a thunk (known as a blast in the vernacular). The statement form can be particularly useful to expose a lexically scoped declaration to the surrounding lexical scope without "trapping" it inside a block.

Hence these declare the same variables with the same scope as the preceding example, but run the statements as a whole at the indicated time:

    BEGIN my $compiletime = now;
    ENTER our $temphandle = maketemp();

(Note, however, that the value of a variable calculated at compile time may not persist under run-time cloning of any surrounding closure.)

Most of the non-value-producing phasers may also be so used:

    END say my $accumulator;

Note, however, that

    END say my $accumulator = 0;

sets the variable to 0 at END time, since that is when the "my" declaration is actually executed. Only argumentless phasers may use the statement form. This means that CATCH and CONTROL always require a block, since they take an argument that sets $_ to the current topic, so that the innards are able to behave as a switch statement. (If bare statements were allowed, the temporary binding of $_ would leak out past the end of the CATCH or CONTROL, with unpredictable and quite possibly dire consequences. Exception handlers are supposed to reduce uncertainty, not increase it.)

Code that is generated at run time can still fire off CHECK and INIT phasers, though of course those phasers can't do things that would require travel back in time. You need a wormhole for that.

The compiler is free to ignore LINK phasers compiled at run time since they're too late for the application-wide linking decisions.

Some of these phasers also have corresponding traits that can be set on variables. These have the advantage of passing the variable in question into the closure as its topic:

    our $h will enter { .rememberit() } will undo { .forgetit() };

Only phasers that can occur multiple times within a block are eligible for this per-variable form.

Apart from CATCH and CONTROL, which can only occur once, most of these can occur multiple times within the block. So they aren't really traits, exactly--they add themselves onto a list stored in the actual trait. So if you examine the ENTER trait of a block, you'll find that it's really a list of phasers rather than a single phaser. In general, initializing phasers execute in order declared, while finalizing phasers execute in the opposite order. When phasers are in different modules, the INIT and END phasers are treated as if declared at use time in the using module. (It is erroneous to depend on this order if the module is used more than once, however, since the phasers are only installed the first time they're noticed.)

The semantics of INIT and once are not equivalent to each other in the case of cloned closures. An INIT only runs once for all copies of a cloned closure. A once runs separately for each clone, so separate clones can keep separate state variables:

    our $i = 0;
    ...
    $func = once { state $x { $x = $i++ }; dostuff($i) };

But state automatically applies "once" semantics to any initializer, so this also works:

    $func = { state $x = $i++; dostuff($i) }

Each subsequent clone gets an initial state that is one higher than the previous, and each clone maintains its own state of $x, because that's what state variables do.

Even in the absence of closure cloning, INIT runs before the mainline code, while once puts off the initialization till the last possible moment, then runs exactly once, and caches its value for all subsequent calls (assuming it wasn't called in sink context, in which case the once is evaluated once only for its side effects). In particular, this means that once can make use of any parameters passed in on the first call, whereas INIT cannot.

All of these phaser blocks can see any previously declared lexical variables, even if those variables have not been elaborated yet when the closure is invoked (in which case the variables evaluate to an undefined value.)

Note: Apocalypse 4 confused the notions of PRE/POST with ENTER/LEAVE. These are now separate notions. ENTER and LEAVE are used only for their side effects. PRE and POST return boolean values which, if false, trigger a runtime exception. KEEP and UNDO are just variants of LEAVE, and for execution order are treated as part of the queue of LEAVE phasers.

It is conjectured that PRE and POST submethods in a class could be made to run as if they were phasers in any public method of the class. This feature is awaiting further exploration by means of a ClassHOW extension.

FIRST, NEXT, and LAST are meaningful only within the lexical scope of a loop, and may occur only at the top level of such a loop block. A NEXT executes only if the end of the loop block is reached normally, or an explicit next is executed. In distinction to LEAVE phasers, a NEXT phaser is not executed if the loop block is exited via any exception other than the control exception thrown by next. In particular, a last bypasses evaluation of NEXT phasers.

[Note: the name FIRST used to be associated with state declarations. Now it is associated only with loops. See the once above for state semantics.]

Except for CATCH and CONTROL phasers, which run while an exception is looking for a place to handle it, all block-leaving phasers wait until the call stack is actually unwound to run. Unwinding happens only after some exception handler decides to handle the exception that way. That is, just because an exception is thrown past a stack frame does not mean we have officially left the block yet, since the exception might be resumable. In any case, exception handlers are specified to run within the dynamic scope of the failing code, whether or not the exception is resumable. The stack is unwound and the phasers are called only if an exception is not resumed.

So LEAVE phasers for a given block are necessarily evaluated after any CATCH and CONTROL phasers. This includes the LEAVE variants, KEEP and UNDO. POST phasers are evaluated after everything else, to guarantee that even LEAVE phasers can't violate postconditions. Likewise PRE phasers fire off before any ENTER or FIRST (though not before BEGIN, CHECK, LINK, or INIT, since those are done at compile or process initialization time).

The POST block can be defined in one of two ways. Either the corresponding POST is defined as a separate phaser, in which case PRE and POST share no lexical scope. Alternately, any PRE phaser may define its corresponding POST as an embedded phaser block that closes over the lexical scope of the PRE.

If exit phasers are running as a result of a stack unwind initiated by an exception, this information needs to be made available. In any case, the information as to whether the block is being exited successfully or unsuccessfully needs to be available to decide whether to run KEEP or UNDO blocks (also see "Definition of Success"). How this information is made available is implementation dependent.

An exception thrown from an ENTER phaser will abort the ENTER queue, but one thrown from a LEAVE phaser will not. The exceptions thrown by failing PRE and POST phasers cannot be caught by a CATCH in the same block, which implies that POST phaser are not run if a PRE phaser fails.

If a POST fails or any kind of LEAVE block throws an exception while the stack is unwinding, the unwinding continues and collects exceptions to be handled. When the unwinding is completed all new exceptions are thrown from that point.

For phasers such as KEEP and POST that are run when exiting a scope normally, the return value (if any) from that scope is available as the current topic within the phaser. (It is presented as a argument, that is, either as parcel or an object that can stand alone in a list. In other words, it's exactly what return is sending to the outside world in raw form, so that the phaser doesn't accidentally impose context prematurely.)

The topic of the block outside a phaser is still available as OUTER::<$_>. Whether the return value is modifiable may be a policy of the phaser in question. In particular, the return value should not be modified within a POST phaser, but a LEAVE phaser could be more liberal.

Any phaser defined in the lexical scope of a method is a closure that closes over self as well as normal lexicals. (Or equivalently, an implementation may simply turn all such phasers into submethods whose primed invocant is the current object.)

Statement parsing

In this statement:

    given EXPR {
        when EXPR { ... }
        when EXPR { ... }
        ...
    }

parentheses aren't necessary around EXPR because the whitespace between EXPR and the block forces the block to be considered a block rather than a subscript, provided the block occurs where an infix operator would be expected. This works for all control structures, not just the new ones in Perl 6. A top-level bare block is always considered a statement block if there's a term and a space before it:

    if $foo { ... }
    elsif $bar { ... }
    else { ... }
    while $more { ... }
    for 1..10 { ... }

You can still parenthesize the expression argument for old times' sake, as long as there's a space between the closing paren and the opening brace. (Otherwise it will be parsed as a hash subscript.)

Note that the parser cannot intuit how many arguments a list operator is taking, so if you mean 0 arguments, you must parenthesize the argument list to force the block to appear after a term:

    if caller {...}    # WRONG, parsed as caller({...})
    if caller() {...}  # okay
    if (caller) {...}  # okay

Note that common idioms work as expected though:

    for map { $^a + 1 }, @list { .say }

Unless you are parsing a statement that expects a block argument, it is illegal to use a bare closure where an operator is expected because it will be considered to be two terms in row. (Remove the whitespace if you wish it to be a postcircumfix.)

Anywhere a term is expected, a block is taken to be a closure definition (an anonymous subroutine). If a closure has arguments, it is always taken as a normal closure. (In addition to standard formal parameters, placeholder arguments also count, as do the underscore variables. Implicit use of $_ with .method also counts as an argument.)

However, if an argumentless closure is empty, or appears to contain nothing but a comma-separated list starting with a pair or a hash (counting a single pair or hash as a list of one element), the closure will be immediately executed as a hash composer, as if called with .().

    $hash = { };
    $hash = { %stuff };
    $hash = { "a" => 1 };
    $hash = { "a" => 1, $b, $c, %stuff, @nonsense };
    $code = { %_ };                            # use of %_
    $code = { "a" => $_ };                     # use of $_
    $code = { "a" => 1, $b, $c, %stuff, @_ };  # use of @_
    $code = { ; };
    $code = { @stuff };
    $code = { "a", 1 };
    $code = { "a" => 1, $b, $c ==> print };

If you wish to be less ambiguous, the hash list operator will explicitly evaluate a list and compose a hash of the returned value, while sub or -> introduces an anonymous subroutine:

    $code = -> { "a" => 1 };
    $code = sub { "a" => 1 };
    $hash = hash("a" => 1);
    $hash = hash("a", 1);

Note that the closure in a map will never be interpreted as a hash, since such a closure always takes arguments, and use of placeholders (including underscore variables) is taken as evidence of arguments.

If a closure is the right argument of the dot operator, the closure is interpreted as a hash subscript.

    $code = {$x};       # closure because term expected
    if $term{$x}        # subscript because postfix expected
    if $term {$x}       # expression followed by statement block
    if $term.{$x}       # valid subscript with dot
    if $term\  {$x}     # valid subscript with "unspace"

Similar rules apply to array subscripts:

    $array = [$x];      # array composer because term expected
    if $term[$x]        # subscript because postfix expected
    if $term [$x]       # syntax error (two terms in a row)
    if $term.[$x]       # valid subscript with dot
    if $term\  [$x]     # valid subscript with "unspace"

And to the parentheses delimiting function arguments:

    $scalar = ($x);     # grouping parens because term expected
    if $term($x)        # function call because operator expected
    if $term ($x)       # syntax error (two terms in a row)
    if $term.($x)       # valid function call with explicit dot deref
    if $term\  .($x)    # valid function call with "unspace" and dot

Outside of any kind of expression brackets, a final closing curly on a line (not counting whitespace or comments) always reverts to the precedence of semicolon whether or not you put a semicolon after it. (In the absence of an explicit semicolon, the current statement may continue on a subsequent line, but only with valid statement continuators such as else that cannot be confused with the beginning of a new statement. Anything else, such as a statement modifier (on, say, a loop statement) must continue on the same line, unless the newline be escaped using the "unspace" construct--see S02.)

Final blocks on statement-level constructs always imply semicolon precedence afterwards regardless of the position of the closing curly. Statement-level constructs are distinguished in the grammar by being declared in the statement_control category:

    macro statement_control:<if> ($expr, &ifblock) {...}
    macro statement_control:<while> ($expr, &whileblock) {...}
    macro statement_control:<BEGIN> (&beginblock) {...}

Statement-level constructs may start only where the parser is expecting the start of a statement. To embed a statement in an expression you must use something like do {...} or try {...}.

    $x =  do { given $foo { when 1 {2}; when 3 {4} } } + $bar;
    $x = try { given $foo { when 1 {2}; when 3 {4} } } + $bar;

The existence of a statement_control:<BEGIN> does not preclude us from also defining a prefix:<BEGIN> that can be used within an expression:

    macro prefix:<BEGIN> (&beginblock) { beginblock().repr }

Then you can say things like:

    $recompile_by = BEGIN { time } + $expiration_time;

But statement_control:<BEGIN> hides prefix:<BEGIN> at the start of a statement. You could also conceivably define a prefix:<if>, but then you may not get what you want when you say:

    die if $foo;

since prefix:<if> would hide statement_modifier:<if>.

Built-in statement-level keywords require whitespace between the keyword and the first argument, as well as before any terminating loop. In particular, a syntax error will be reported for C-isms such as these:

    if(...) {...}
    while(...) {...}
    for(...) {...}

Definition of Success

Hypothetical variables are somewhat transactional--they keep their new values only on successful exit of the current block, and otherwise are rolled back to their original values.

It is, of course, a failure to leave the block by propagating an error exception, though returning a defined value after catching an exception is okay.

In the absence of error exception propagation, a successful exit is one that returns a defined value or parcel. (A defined parcel may contain undefined values.) So any Perl 6 function can say

    fail "message";

and not care about whether the function is being called in item or list context. To return an explicit scalar undef, you can always say

    return Mu;          # like "return undef" in Perl 5

Then in list context, you're returning a list of length 1, which is defined (much like in Perl 5). But generally you should be using fail in such a case to return an exception object. In any case, returning an unthrown exception is considered failure from the standpoint of let. Backtracking over a closure in a regex is also considered failure of the closure, which is how hypothetical variables are managed by regexes. (And on the flip side, use of fail within a regex closure initiates backtracking of the regex.)

When is a closure not a closure

Everything is conceptually a closure in Perl 6, but the optimizer is free to turn unreferenced closures into mere blocks of code. It is also free to turn referenced closures into mere anonymous subroutines if the block does not refer to any external lexicals that should themselves be cloned. (When we say "clone", we mean the way the system takes a snapshot of the routine's lexical scope and binds it to the current instance of the routine so that if you ever use the current reference to the routine, it gets the current snapshot of its world in terms of the lexical symbols that are visible to it.)

All remaining blocks are conceptually cloned into closures as soon as the lexical scope containing them is entered. (This may be done lazily as long as consistent semantics are preserved, so a block that is never executed and never has a reference taken can avoid cloning altogether. Execution or reference taking forces cloning in this case--references are not allowed to be lazily cloned, since no guarantee can be made that the scope needed for cloning will remain in existence over the life of the reference.)

In particular, package subroutines are a special problem when embedded in a changing lexical scope (when they make reference to it). The binding of such a definition to a name within a symbol table counts as taking a reference, so at compile time there is an initial binding to the symbol table entry in question. For "global" bindings to symbol tables visible at compile time, this binds to the compile-time view of the lexical scopes. (At run-time, the initial run-time view of these scopes is copied from the compiler's view of them, so that initializations carry over, for instance.) At run time, when such a subroutine is cloned, an additional binding is done at clone time to the same symbol table entry that the original was bound to. (The binding is not restored on exit from the current lexical scope; this binding records the last cloning, not the currently in-use cloning, so any use of the global reference must take into consideration that it is functioning only as a cache of the most recent cloning, not as a surrogate for the current lexical scope.)

Matters are more complicated if the package in question is lexically defined. In such cases, the package must be cloned as if it were a sub on entry to the corresponding lexical scope. All runtime instances of a single package declaration share the same set of compile-time declared functions, however, the runtime instances can have different lexical environments as described in the preceding paragraph. If multiple conflicting definitions of a sub exist for the same compile-time package, an error condition exists and behavior is not specified for Perl 6.0.

Methods in classes behave functionally like package subroutines, and have the same binding behavior if the classes are cloned. Note that a class declaration, even an augment, is fundamentally a compile-time operation; composition only happens once and the results are recorded in the prototype class. Runtime typological manipulations are limited to reseating OUTER:: scopes of methods.

Lexical names do not share this problem, since the symbol goes out of scope synchronously with its usage. Unlike global subs, they do not need a compile-time binding, but like global subs, they perform a binding to the lexical symbol at clone time (again, conceptually at the entry to the outer lexical scope, but possibly deferred.)

    sub foo {
        # conceptual cloning happens to both blocks below
        my $x = 1;
        my sub bar { print $x }         # already conceptually cloned, but can be lazily deferred
        my &baz := { bar(); print $x }; # block is cloned immediately, forcing cloning of bar
        my $code = &bar;                # this would also force bar to be cloned
        return &baz;
    }

In particular, blocks of inline control flow need not be cloned until called. [Note: this is currently a potential problem for user-defined constructs, since you have to take references to blocks to pass them to whatever is managing the control flow. Perhaps the laziness can be deferred through Captures to binding time, so a slurpy of block refs doesn't clone them all prematurely. On the other hand, this either means the Capture must be smart enough to keep track of the lexical scope it came from so that it can pass the info to the cloner, or it means that we need some special fat not-cloned-yet references that can carry the info lazily. Neither approach is pretty.]

Some closures produce Block objects at compile time that cannot be cloned, because they're not attached to any runtime code that can actually clone them. BEGIN, CHECK, LINK, INIT, and END blocks fall into this category. Therefore you can't reliably refer to run-time variables from these closures even if they appear to be in the scope. (The compile-time closure may, in fact, see some kind of permanent copy of the variable for some storage classes, but the variable is likely to be undefined when the closure is run in any case.) It's only safe to refer to package variables and file-scoped lexicals from such a routine.

On the other hand, it is required that CATCH and LEAVE blocks be able to see transient variables in their current lexical scope, so their cloning status depends at least on the cloning status of the block they're in.

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