=begin pod

=TITLE Phasers

=SUBTITLE Program execution phases and corresponding phaser blocks

The lifetime (execution timeline) of a program is broken up into phases. A
I<phaser> is a block of code called during a specific execution phase.

=head1 Phasers

A phaser 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 I<phasers>, since they generally mark the transition from one phase of
computing to another. For instance, a C<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.

Here is a summary:

      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

Phasers marked with a C<*> 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();

Most of these phasers will take either a block or a function reference. The
statement form can be particularly useful to expose a lexically scoped
declaration to the surrounding lexical scope without "trapping" it inside a
block.

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

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.

The topic of the block outside a phaser is still available as C<< 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
C<POST> phaser, but a C<LEAVE> phaser could be more liberal.

Any phaser defined in the lexical scope of a method is a closure that closes
over C<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.)

When multiple phasers are scheduled to run at the same moment, the general
tiebreaking principle is that initializing phasers execute in order declared,
while finalizing phasers execute in the opposite order, because setup and
teardown usually want to happen in the opposite order from each other.

=head2 Execution Order

    Compilation Begins

      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
    COMPOSE {...} #  when a role is composed into a class

    Execution Begins

       INIT {...} #  at run time, ASAP, only ever runs once

    Before block execution begins

        PRE {...} #  assert precondition at every block entry, before ENTER

    Loop execution begins

      FIRST {...} #  at loop initialization time, before any ENTER

    Block execution begins

      ENTER {...} #  at every block entry time, repeats on loop blocks.

    Exception maybe happens

      CATCH {...} #  catch exceptions, before LEAVE
    CONTROL {...} #  catch control exceptions, before LEAVE

    End of loop, either continuing or finished

       NEXT {...} #  at loop continuation time, before any LEAVE
       LAST {...} #  at loop termination time, after any LEAVE

    End of block

      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

    Post-condition for block

       POST {...} #  assert postcondition at every block exit, after LEAVE

    Program terminating

        END {...} #  at run time, ALAP, only ever runs once

=head1 Program Execution Phasers

=head2 BEGIN

Runs at compile time, As Soon As Possible, only runs once.

Can have a return value that is provided even in later phases.

=head2 CHECK

Runs at compile time, As Last As Possible, only runs once.

Can have a return value that is provided even in later phases.

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

=head2 LINK

Runs at link time, As Last As Possible, only runs once.

Can have a return value that is provided even in later phases.

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

=head2 INIT

Runs after compilation during main execution, As Soon As Possible, only runs
once.

Can have a return value that is provided even in later phases.

When phasers are in different modules, the C<INIT> and C<END> phasers are
treated as if declared at C<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.)

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

An C<INIT> only runs once for all copies of a cloned closure.

=head2 END

Runs after compilation during main execution, As Last As Possible, only runs
once.

When phasers are in different modules, the C<INIT> and C<END> phasers are
treated as if declared at C<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.)

=head1 Block Phasers

Execution in the context of a block has it's own phases.

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.

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 C<ENTER> trait of a block, you'll find that it's really a list of
phasers rather than a single phaser.

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.)

=head2 ENTER

Runs at every block entry time, repeats on loop blocks.

Can have a return value that is provided even in later phases.

An exception thrown from an C<ENTER> phaser will abort the C<ENTER> queue, but
one thrown from a C<LEAVE> phaser will not.

=head2 LEAVE

Runs at every block exit time (even stack unwinds from exceptions).

So C<LEAVE> phasers for a given block are necessarily evaluated after any
C<CATCH> and C<CONTROL> phasers. This includes the C<LEAVE> variants, C<KEEP>
and C<UNDO>. C<POST> phasers are evaluated after everything else, to guarantee
that even C<LEAVE> phasers can't violate postconditions.

An exception thrown from an C<ENTER> phaser will abort the C<ENTER> queue, but
one thrown from a C<LEAVE> phaser will not.

If a C<POST> fails or any kind of C<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.

=head2 KEEP

Runs at every successful block exit, as part of the LEAVE queue (shares the
same order of execution).

For phasers such as C<KEEP> and C<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.

=head2 UNDO

Runs at every unsuccessful block exit, as part of the LEAVE queue (shares the
same order of execution).

=head2 PRE

Asserts a precondition at every block entry. Runs before the ENTER phase.

C<PRE> phasers fire off before any C<ENTER> or C<FIRST>.

The exceptions thrown by failing C<PRE> and C<POST> phasers cannot be caught by
a C<CATCH> in the same block, which implies that C<POST> phaser are not run if
a C<PRE> phaser fails.

=head2 POST

Asserts a postcondition at every block entry. Runs after the LEAVE phase.

For phasers such as C<KEEP> and C<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.

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

If a C<POST> fails or any kind of C<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.

The exceptions thrown by failing C<PRE> and C<POST> phasers cannot be caught by
a C<CATCH> in the same block, which implies that C<POST> phaser are not run if
a C<PRE> phaser fails.

=head1 Loop Phasers

C<FIRST>, C<NEXT>, and C<LAST> are meaningful only within the lexical scope of
a loop, and may occur only at the top level of such a loop block.

=head2 FIRST

Runs at loop initialization, before ENTER.

Can have a return value that is provided even in later phases.

=head2 NEXT

Runs when loop is continued (either through C<next> or because you got to the
bottom of the loop and are looping back around), before LEAVE.

A C<NEXT> executes only if the end of the loop block is reached normally, or an
explicit C<next> is executed. In distinction to C<LEAVE> phasers, a C<NEXT>
phaser is not executed if the loop block is exited via any exception other than
the control exception thrown by C<next>. In particular, a C<last> bypasses
evaluation of C<NEXT> phasers.

=head2 LAST

Runs when loop is aborted (either through C<last>, or C<return>, or because you
got to the bottom of the loop and are done), after LEAVE.

=head1 Exception Handling Phasers

=head2 CATCH

Runs when an exception is raised by the current block, before the LEAVE phase.

=head2 CONTROL

Runs when a control exception is raised by the current block, before the LEAVE
phase.

=head1 Object Phasers

=head2 COMPOSE

Runs when a role is composed into a class.

=end pod
