Fri, 28 Nov 2008

Unicode


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NAME

"Perl 5 to 6" Lesson 17 - Unicode

SYNOPSIS

  (none)    

DESCRIPTION

Perl 5's Unicode model suffers from a big weakness: it uses the same type for binary and for text data. For example if your program reads 512 bytes from a network socket, it is certainly a byte string. However when (still in Perl 5) you call uc on that string, it will be treated as text. The recommended way is to decode that string first, but when a subroutine receives a string as an argument, it can never surely know if it had been encoded or not, ie if it is to be treated as a blob or as a text.

Perl 6 on the other hand offers the type buf, which is just a collection of bytes, and Str, which is a collection of logical characters.

Logical character is still a vague term. To be more precise a Str is an object that can be viewed at different levels: Byte, Codepoint (anything that the Unicode Consortium assigned a number to is a codepoint), Grapheme (things that visually appear as a character) and CharLingua (language defined characters).

For example the string with the hex bytes 61 cc 80 consists of three bytes (obviously), but can also be viewed as being consisting of two codepoints with the names LATIN SMALL LETTER A (U+0041) and COMBINING GRAVE ACCENT (U+0300), or as one grapheme that, if neither my blog software nor your browser kill it, looks like this: à.

So you can't simply ask for the length of a string, you have to ask for a specific length:

  $str.bytes;
  $str.codes;
  $str.graphs;

There's also method named chars, which returns the length in the current Unicode level (which can be set by a pragma like use bytes, and which defaults to graphemes).

In Perl 5 you sometimes had the problem of accidentally concatenating byte strings and text strings. If you should ever suffer from that problem in Perl 6, you can easily identify where it happens by overloading the concatenation operator:

  sub GLOBAL::infix:<~> is deep (Str $a, buf $b)|(buf $b, Str $a) {
      die "Can't concatenate text string «"
          ~ $a.encode("UTF-8")
            "» with byte string «$b»\n";
  }

Encoding and Decoding

The specification of the IO system is very basic and does not yet define any encoding and decoding layers, which is why this article has no useful SYNOPSIS section. I'm sure that there will be such a mechanism, and I could imagine it will look something like this:

  my $handle = open($filename, :r, :encoding<UTF-8>);

Regexes and Unicode

Regexes can take modifiers that specify their Unicode level, so m:codes/./ will match exactly one codepoint. In the absence of such modifiers the current Unicode level will be used.

Character classes like \w (match a word character) behave accordingly to the Unicode standard. There are modifiers that ignore case (:i) and accents (:a), and modifiers for the substitution operators that can carry case information to the substitution string (:samecase and :sameaccent, short :ii, :aa).

MOTIVATION

It is quite hard to correctly process strings with most tools and most programming languages these days. Suppose you have a web application in perl 5, and you want to break long words automatically so that they don't mess up your layout. When you use naive substr to do that, you might accidentally rip graphemes apart.

Perl 6 will be the first mainstream programming language with built in support for grapheme level string manipulation, which basically removes most Unicode worries, and which (in conjunction with regexes) makes Perl 6 one of the most powerful languages for string processing.

The separate data types for text and byte strings make debugging and introspection quite easy.

SEE ALSO

http://perlcabal.org/syn/S32/Str.html

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