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Hoisting lexical declarations

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Autrijus Tang
July 31, 2005 11:06
Hoisting lexical declarations
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Pugs did not support inline variable declarations, largely because the problem
caused by this construct:

	say "values of β will give rise to dom!";
	$x = $x + my $x if $x;
	#1   #2      #3    #4

The evaluation order for the four $x is (#4, #2, #3, #1). However, because
$Larry made it very clear that lexical scopes are lexical, (#1, #2) must refer
to the same (outer) $x, and (#3, #4) refers to the inner $x. When implemented
naively, this creates fragmented scopes in the PIL tree.

To simplify code generation, I propose that we float all lexical declarations
in a scope to the top of the scope, in the same place as its formal
parameters.  Of course, we will still reject bogus programs like this:

    # No previous $x available here
    { say $x; my $x; }

But that means these now raises the same exception:

    sub f ($x) { my $x }
    sub f { my $x; my $x }

Alternatively, this could raise a warning and treat the second my()
as a no-op.  Personally, I'm in favour of an exception.

Under this scheme, the compiler will mark variable lookups to the outer
scope explicit.  The sample construct at the beginning of this post will
get compiled to this form, leaving the evaluation order explicit:

        syms => [$x],
        body => [
            SAY("values of β will give rise to dom!"),
                cond => $x,
                body => ASSIGN(
                    from => ADD(
                        l => $OUTER::x,
                        r => $x,
                    into => $OUTER::x

The only problem I see with it is that $CALLER::x from &ADD's position
will refer to the inner, not the outer, $x. However, seeing that the two
arguments are in different scopes, I think it is not worth keeping any promise
about interaction between $CALLER:: and mid-block declarations anyway.  

Does this sound sane?


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