Re^4: A short whishlist of Perl5 improvements leaping to Perl7 (Thought Experiment No.1)

let's try a proposal as thought experiment and discuss it

short version:*

basically only file and function scope are automatic.
all deeper nested scopes need to be explicit. Implicit effects there - if allowed - belong to the surrounding file/function scope.
nested subs are tricky

1. Any assignment to an undeclared new variable implies a "implicit" my when in

file-scope (outside any sub)
$x = 1 ==> my $x =1
function-scope
sub foo { $x = 1; ... } ==> sub foo { my $x =1; ... }
loop-header but NOT body
for $x (@a) {...} ==> for my $x (@a) {...}
some commands like open *
open $fh, ... ==> open my $fh, ...

2. explicit my are still optionally allowed, our and state keep their semantics

3. No hoisting, a variable accessed before an implicit my still belongs to the upper scope and needs to be declared there

4. loop-bodies and naked blocks (technically the same) follow the old rules

to restrict the scope of a var you'll still need an explicit my
an implicit my (i.e. assignment to a new var) belongs to the outer scope, starting with the block
{ $x=42 }; print $x; => 42

5. ( ??? ) nested/anonymous subs are tricky, when it comes to closed over vars

if they follow rule 1, than we need a declaration alike Py's nonlocal ... maybe its or over to mark vars avoiding implicit my
if they are exempt from rule 1 and rather act like 4, we get a break in symmetry
another solution would be a new keyword fun to create a sub with auto-mine while keeping sub explicit.

6. ( ??? ) functional blocks like in map {BLOCK} are technically anonymous subs °

see point 5

7. ( ??? ) class-variables are a current issue in proposed new OOP models.

8. special global vars are exempt from implicit my.

9. Last but not least automine activates strict

So ...

Did I forget a case?
Do you need examples?
Please inform yourself about closures before judging.

Cheers Rolf
_{(addicted to the Perl Programming Language :)

Wikisyntax for the Monastery}

°) actually not in the case of map and grep but for all of List::Util , try return to see the difference

*) updated

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Re^5: A short whishlist of Perl5 improvements leaping to Perl7 (Thought Experiment No.2)
by LanX (Saint) on Nov 28, 2020 at 12:48 UTC

1. any assignment to a new variable leads to an implicit my but only if their was no previous explicit declaration in a surrounding scope. This will replace former rules 1,4 and 5 all others still apply

Hence we get the following 4 cases for explicit declarations with my or our

automine effect comment

{ $x = 1; { $x = 2; } }
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{ my $x = 1; { my $x = 2; } }
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not strict w/o automine

{ $x = 1; { my $x = 2; } }
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{ my $x = 1; { my $x = 2; } }
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not strict w/o automine

{ my $x = 1; { $x = 2; } }
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{ my $x = 1; { $x = 2; } }
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backwards compatible

{ my $x = 1; { my $x = 2; } }
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{ my $x = 1; { my $x = 2; } }
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backwards compatible

Surprises and pitfalls:

automine effect comment

sub foo { if (1) { $x = 2; } print $x; }
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sub foo { if (1) { my $x = 2; } print $x; # undeclared }
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Doesn't compile
Surprising for Pythonistas

sub add { $x = 0; for (1..9) { $x = $x + $_; } print $x; }
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sub add { my $x = 0; for (1..9) { my $x = $x + $_; } print $x; # prints 0 }
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Unexpected result!
Needs an explicit my $x = 0 at init to avoid implicit my

This is a very Perlish approach and avoids many edge cases.

Of course this laziness comes with a price, introducing a new explicit declaration on file scope has an effect at the distance and could silently break subroutines

use automine
# our $var;

... 

sub foo {
    $var = shift; # not 'my $var' if explicit 'our $var' exists 
    ...
}
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That's why this theoretical pragma must be optional. (Of course scoping the our inside a tight block would also prevent this.)

Cheers Rolf
_{(addicted to the Perl Programming Language :)

Wikisyntax for the Monastery}

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