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Re: BEGIN {} question
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From:
ToddAndMargo via perl6-users
Date:
August 29, 2022 02:38
Subject:
Re: BEGIN {} question
Message ID:
82c9bd26-e70d-e3ca-9eff-1916314553d5@zoho.com
On 8/28/22 19:11, Bruce Gray wrote:
>
>
>> On Aug 28, 2022, at 5:58 PM, ToddAndMargo via perl6-users <perl6-users@perl.org> wrote:
>>
>> Hi All,
>>
>> I am thinking of using
>>
>> BEGIN {}
>>
>> to fire up a splash screen (libnotify).
>>
>> Question: is what happens between the brackets
>> isolated from the rest of the code? If I set
>> variable values or declare variables, are they
>> wiped out, etc.?
>>
>> Many thanks,
>> -T
>
> BEGIN blocks create a lexical scope, because they are *blocks*, so any variables that you declare within the block don't exist outside the block.
>
> Variables that you define in the lexical scope *surrounding* the BEGIN block can have their values set inside the BEGIN block, and those values will be retained after BEGIN ends.
>
> my $a_var;
> sub do_something ( ) {
> say "did something! By the way: ", (:$a_var), ' inside a sub called from the BEGIN block, because the var is shared between them (same lexical scope).';
> }
> BEGIN {
> $a_var = 42;
> my $b_var = 11;
> say "a_var is $a_var within the BEGIN block";
> say "b_var is $b_var within the BEGIN block";
> do_something();
> }
> say "a_var is still $a_var outside the BEGIN block";
> # say "b_var is still $b_var outside the BEGIN block"; # Commented out, because illegal!
>
> Output:
> a_var is 42 within the BEGIN block
> b_var is 11 within the BEGIN block
> did something! By the way: a_var => 42 inside a sub called from the BEGIN block, because the var is shared between them (same lexical scope).
> a_var is still 42 outside the BEGIN block
>
Hi Bruce,
Thank you! I understand now.
I was channeling my old Modula2 days, where
everything had a BEGIN and an END. I did not
realize BEGIN was a "name". A special name
of a subroutine that would run before compile
was complete.
I am now thinking of firing off a call to libnotify
with a delayed close out time to simulate a splash
screen.
Question, would BEGIN go at the top or the bottom
of my code? Seems the compiler would hit it first
at the top, but I do not know if it makes a full
pass of everything before firing off the BEGIN.
-T
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