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Sorry, I don't buy this proof. In the last code block:

sub halts { my $machine = shift; my $input = shift; is_whatever_nullary( qq{ run_turing_equivalent("\Q$machine\E", "\Q$input\E"); sub whatever() {}; } ) }

is_whatever_nullary() can certainly easily return a true value without having to execute run_turing_equivalent(). The whatever prototype is a compile-time property and the run-time impact of whatever run_turing_equivalent() does will not change the fact that whatever() is "nullary" at compile time.

But it is quite obvious that Perl code can't be reliably parsed without running Perl code by the simple example of:

BEGIN { if( 0.5 < rand() ) { eval "sub whatever() { }; 1" or die $@; } else { eval "sub whatever { }; 1" or die $@; } } whatever / 25 ; # / ; die "this dies!";

You can, of course, replace the above conditional with some high-falutin' comp sci construct of your choosing. But I find that such just detracts from the obviousness. You have to run "0.5 < rand()" in order to parse the last line of the above Perl code (which perl can parse differently each time that it is run, as shown below).

> perl -MO=Deparse above.pl # ... whatever / 25; - syntax OK > perl -MO=Deparse above.pl # ... whatever(/ 25 ; # /); die 'this dies!'; - syntax OK

- tye        


In reply to Re: Perl Cannot Be Parsed: A Formal Proof (meh) by tye
in thread Perl Cannot Be Parsed: A Formal Proof by Jeffrey Kegler

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