Perl 6 phase interleaving

interleaving.pod6

Perl 6 first compiles code, and then runs it.

The C<BEGIN> phaser interleaves a run time phase into the
compilation phase, that is, the code inside the C<BEGIN>
phaser is first compiled, then run, and then the rest of the
compilation continues.

In the hypothetical code example

    A;
    BEGIN { B }
    C;

the phases are:

=item compilation of A
=item compilation of B
=item run time of B
=item compilation of C
=item run time of A
=item run time of C

The same happens when C<use>ing a module that is not precompiled; in a code
like C<use SomeModule;>, the C<use> statement runs at compile time, which
first compiles C<SomeModule>, runs the mainline of C<SomeModule>, and then
continues compiling the rest of the program that contains the C<use> statement.

The inverse of this operation is the C<EVAL> function, which compiles
and then runs code during the run time phase. For instance the phases of

    A;
    EVAL 'B';
    C

are

=item compilation of the whole program
=item run time of A
=item run time of EVAL
=item compilation time of B
=item run time of B
=item run time of C

Interleaving of run time and compile time can be arbitrarily nested. If
you C<use> a module that C<use>s another module, that already constitutes two
levels of nesting.