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Currency symbols in variable names

by pme (Prior)
on Aug 25, 2019 at 10:32 UTC ( #11104977=perlquestion: print w/replies, xml ) Need Help??

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

Hi Monks,

Can you tell me why this example below does not work in perl v5.28?

use utf8; sub £_to_₯ { ... } my $yen = £_to_₯('1000£');
It is from book http://modernperlbooks.com/books/modern_perl_2016/03-perl-language.html#VW5pY29kZWFuZFN0cmluZ3M by chromatic.

This is my tiny script and its output with diagnostics.

#!/usr/bin/perl use strict; use warnings; use diagnostics; use utf8; use feature qw/say/; binmode STDOUT, ":utf8"; my $€ = 1; say '------------------------';
Output:
Unrecognized character \x{20ac}; marked by <-- HERE after my $<-- HERE + near column 5 at ./uni2.pl line 10 (#1) (F) The Perl parser has no idea what to do with the specified char +acter in your Perl script (or eval) near the specified column. Perhaps +you tried to run a compressed script, a binary program, or a director +y as a Perl program. Uncaught exception from user code: Unrecognized character \x{20ac}; marked by <-- HERE after my $<-- +HERE near column 5 at ./uni2.pl line 10.

Replies are listed 'Best First'.
Re: Currency symbols in variable names
by davido (Cardinal) on Aug 26, 2019 at 05:48 UTC

    A simplistic view of what constitutes a legitimate identifier is any name that matches the following regexp:

    m/\A\[_\p{ID_Start}]\p{XID_Continue}*\z/

    But this has many shortcomings, some of which are described in a StackOverflow article from tchrist (When I read his articles dealing with Unicode I mostly just feel small). It doesn't allow for most punctuation variables described in perlvar, for example... but even discounting those, it's just a start.

    I mention this because the symbols you are using in your identifier will not match that regular expression, despite ID_Start and XID_Continue matching well over a hundred thousand characters. And these aren't punctuation characters described in perlvar either. So for typical use you can probably disqualify them outright.

    One thing of note is that when wielding symbolic refs, nearly any set of characters is legal, including your currency symbols. Case in point:

    use utf8; binmode STDOUT, ':utf8'; no strict 'refs'; my $symbol = '£1%^@"₯'; $$symbol = 42; print "\$$symbol = $$symbol\n"; print "$symbol is found in package ", *{__PACKAGE__ . '::£1%^@"₯'}{PAC +KAGE}, "\n"; print "\$main::${symbol} has a value of ", ${'main::£1%^@"₯'}, "\n";

    Spoiler alert, it works. And notice we're using the £ and ₯ characters.

    Symbol manipulation is something Perl is good at, but that we try to avoid for our own sanity. A language that more fully embraces it is Scheme. And since symbol manipulation is the name of the game for Scheme, naturally that language is fine with ₯ as a function identifier:

    (define ₯ (lambda (x y) (+ x y))) ;Value: |β₯| 1 ]=> (₯ 3 5) ;Value: 8


    Dave

Re: Currency symbols in variable names
by 1nickt (Abbot) on Aug 25, 2019 at 10:55 UTC

    Hi, not all characters may be used in variable names, even under UTF-8. See perldata.

    Try this:

    #!/usr/bin/perl use strict; use warnings; use diagnostics; use utf8; use feature qw/say/; binmode STDOUT, ":utf8"; my $ιlan; say '------------------------';

    Hope this helps!


    The way forward always starts with a minimal test.
      Hi 1nickt,

      It seems to be that unicode characters can be in variable names if they are letters and currency symbols are not considered as letters.

      my $愛 = 4; # correct

      my $s = '★'; # correct

      my $★ = 'star'; # incorrect

      However, it makes sense. What could we do with this?

      sub $_to_₯ { ... }
      Update: This one works ...

      my $元 = 3;

        Hi again, I'm not sure whether What could we do with this? refers to the sub after the question, or the way Perl disallows certain characters in variable names. If the latter, I would suggest that independent of your variable naming issue, you consider extensibility of your program. There doesn't seem to be much advantage to a sub named $_to_₯ as compared to a more generic one that takes arguments:

        use strict; use warnings; use Test::More; use Test::More::UTF8; # loads utf8.pm sub convert_currency { my %map = ( '$₯' => sub { $_[0] * 105.4 }, '₯$' => sub { $_[0] * 0.0095 }, '$€' => sub { $_[0] * 0.89 }, '€$' => sub { $_[0] * 1.12 }, ); my ($from, $to, $amount) = @_; my $converter = $map{"$from$to"}; return sprintf( '%.3f', $converter->($amount) ); } is( convert_currency('$', '₯', 5), '527.000', '$ -> ₯' ); is( convert_currency('€', '$', 42.02), '47.062', '€ -> $' ); is( convert_currency('$', '€', 42.02), '37.398', '$ -> €' ); done_testing;
        $ prove -v monks/11104981.pl monks/11104981.pl .. ok 1 - $ -> ₯ ok 2 - € -> $ ok 3 - $ -> € 1..3 ok All tests successful. Files=1, Tests=3, 0 wallclock secs ( 0.02 usr 0.00 sys + 0.04 cusr + 0.00 csys = 0.06 CPU) Result: PASS

        Hope this helps!


        The way forward always starts with a minimal test.
Re: Currency symbols in variable names
by jcb (Deacon) on Aug 26, 2019 at 00:44 UTC

    While other monks have mentioned the restrictions on variable names and recommended a single conversion function that takes arguments to specify a currency pair, I would suggest using standard currency codes instead of Unicode symbols. The standard codes are all in ASCII, and that has the advantage that a person who does not recognize a particular code can still read it, while an unknown symbol is just "what is that supposed to mean?".

Re: Currency symbols in variable names
by holli (Monsignor) on Aug 25, 2019 at 10:57 UTC
    Did you save your file as UTF-8? Right from the very link you posted:
    To write this code, your text editor must understand UTF-8 and you must save the file with the appropriate encoding. Again, any two programs which communicate with Unicode data must agree on the encoding of that data.


    Update: No, doesn't help.


    holli

    You can lead your users to water, but alas, you cannot drown them.
      hi holli,

      I think this note is about how to keep the encoding in the saved file.

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