Ambiguous '~~' in 'print scalar ~~list'

rsFalse has asked for the wisdom of the Perl Monks concerning the following question:

Hello.

I was playing with '~~' "operator" (perlsecret#Inchworm), which is equivalent to 'scalar()' if parsed as two consecutive unary bit-inverting operators, not as experimental smartmatch op. And there is a case where parser can't understand what do I mean (unary forcing scalar or binary smartmatch):

#!/usr/bin/perl -wl

use strict;

    @_ = qw( a b c );
        
    open my $A, '>', "out.txt";
    
    print $A ~~@_;
    print $A scalar @_;
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STDOUT: empty
STDERR: Smartmatch is experimental at smartmatch_or_scalar.pl line 9.
> cat out.txt
3

In my opinion usual operators (bit-inversion) should be in higher precedence of parsing than experimental operators, but in my code '~~' was interpreted as experimental smartmatch.

Comment on Ambiguous '~~' in 'print scalar ~~list' Download Code

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Re: Ambiguous '~~' in 'print scalar ~~list'
by choroba (Cardinal) on Mar 12, 2019 at 13:54 UTC

print {$A} ~~@_;
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map{substr$_->[0],$_->[1]||0,1}[\*||{},3],[[]],[ref qr-1,-,-1],[{}],[sub{}^*ARGV,3]

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Re^2: Ambiguous '~~' in 'print scalar ~~list'

by tobyink (Canon) on Mar 12, 2019 at 16:16 UTC

Using curly braces around the filehandle is generally good practice anyway. Makes it clearer that you're printing something to $A instead of printing the contents of $A.

toby döt ink

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Re: Ambiguous '~~' in 'print scalar ~~list'
by Athanasius (Archbishop) on Mar 12, 2019 at 13:43 UTC

Hello rsFalse,

Yes, the parser is interpreting ~~ as the smartmatch operator here:

23:30 >perl -MO=Deparse 1983_SoPW.pl
Smartmatch is experimental at 1983_SoPW.pl line 9.
BEGIN { $^W = 1; }
BEGIN { $/ = "\n"; $\ = "\n"; }
use strict;
@_ = ('a', 'b', 'c');
open my $A, '>', 'out.txt';
print $A ~~ \@_;
print $A scalar @_;
1983_SoPW.pl syntax OK

23:30 >
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But if you really want to parse this construction as the so-called “inchworm operator,” just use parentheses — print $A (~~@_); — to disambiguate:

23:32 >perl -MO=Deparse 1983_SoPW.pl
BEGIN { $^W = 1; }
BEGIN { $/ = "\n"; $\ = "\n"; }
use strict;
@_ = ('a', 'b', 'c');
open my $A, '>', 'out.txt';
print $A ~~@_;
print $A scalar @_;
1983_SoPW.pl syntax OK

23:32 >
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As to which interpretation should be given “higher precedence” by the parser: that really depends on the way the two constructs are expected to be used. And in fact the double tilde was never intended to be used as an operator at all. If the smartmatch experiment had been successful (it wasn’t), it would have intentionally introduced a new operator into Perl. So of course the parser is going to treat ~~ as a smartmatch operator wherever that makes syntactic sense — that’s what the experiment was all about.

Hope that helps,

Athanasius <°(((>< contra mundum Iustus alius egestas vitae, eros Piratica,

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