Implementing perl BEGIN blocks

[Leopold Toetsch]

There was some recent discussion [1] [2] on p6l about BEGIN blocks and 
constant, which is executed at compile time too.

Parrot has since quite a time the @IMMEDIATE subroutine pragma, which 
causes execution of subs during compilation. But these subroutines can't 
return a result so it's rather useless now besides testing that printing 
something inside thus sub is really done before a print statement in main.

Here is a plan to implement most of the needed bits.

# constant pi = 4 * atan2(1,1);

translates to

.sub anon_1 @IMMEDIATE, @ANON
    $N0 = atan 1.0, 1.0
    $P0 = new .Float
    $P0 = $N0
    .return ($P0)
.end

During compilation this subroutine is executed, the resulting PMC gets 
frozen and replaces the PMC constant slot of the immediate subroutine.

The constant declaration is then just

   .local pmc pi
   set_p_pc pi, anon_1

or with a shortcut:

   .const pmc pi = anon_1

As PMC constants are already represented as frozen images in PBC this 
would work for nested data strucuters, objects, and their classes too.

Comments welcome,
leo

[1] BEGIN {...} and IO
[2] my $pi is constant = 3;